So first off, caveat (more for other readers) that all of the below is from a strictly competitive/tournament standpoint and not from a more casual/average/non-cheese one.
I'll start with Hearthguard Berzerkers comparing to Vulkites. At first glance it can seem like they each have a place, with vulkites being cheaper bodies and battleline while Hzerkers are more elite. However, with a runefather general Hzerkers become battleline, and Hzerkers get a fnp 'save' of a 4+ (technically it could be a 6+ but realistically that will rarely occur) which means they have effectively double the wounds count since it takes 2 damage on average to cause 1 wound. This means that while vulkites have more bodies they die twice as fast, eliminating some of that advantage. Less models means a smaller footprint so it is harder for the enemy to get as many models in range and easier to focus attacks on a single enemy. However the biggest advantage Hzerkers have is more damage. A lot more. To start, the weapon stats:
-10 vulkite handaxe attacks deal an average of 2.96 damage against a 5+ save, 2.22 damage against a 4+ save.
-10 double-handaxe attacks deal 3.9 damage against a 5+ save, 2.96 damage against a 4+ save.
-10 pick attacks deal 2.77 damage against a 5+ save, 2.22 damage against a 4+ save.
-10 Hzerker poleaxe attacks deal an average of 2.96 damage against a 5+ save, 2.22 damage against a 4+ save, plus 3.33 mortal wounds (6.3 and 5.55 total).
-10 broadaxe attacks deal 7.41 damage against a 5+ save, 5.93 damage against a 4+ save (we can see broadaxe is the better weapon option).
There are several other factors to consider. Vulkites cost less points; 3 vulkites cost the same as 2 Hzerkers. But even increasing the damage values of the vulkites by 50% to compensate, the hearthguard still do significantly more. And that is before factoring in hearthguard having a 2" melee range instead of 1". This is huge for 32mm infantry, from experience I can say it works out to a 30-man vulkite unit reasonably having 10-14 models in range while an equivalent 20-man Hzerker unit is likely to have 16-19. This puts the Hzerkers WAY higher in damage output for the same cost. Vulkites do have a few extra benefits: twice as many throwing axe attacks (until they start to die), slingshields on the charge and the 1/game attack on death ability. But those do not raise them even close to how much more damage Hzerkers put out.
And that is not even everything; buffs (of which you have many) grant a larger increase in damage when used on Hzerkers than vulkites. Hzerkers also get a horde discount while vulkites do not, changing the above ration to be more in the former's favor. Finally, being able to go twice in close combat gained from Lords of the Lodge (which you will be taking) means there is essentially no equivalence between the two at all.
So in summary, when looking at competitive builds only take Hzerkers with broadaxes and always have a Runefather general to make them battleline. No other melee troop option is as good (sidenote: this took me 45 minutes to work out and type up, there is no excuse for why GW and/or the playtesters could not have done so). Note that auric hearthguard, while not as effective in strict damage dealing, fill a support role Hzerkers do not and so are excluded from this comparison.
Posting this now, will come back with more later.
Automatically Appended Next Post:
Alrighty, into actual listbuilding.
Vostarg. The bonus to charges, the round one speed boost, and the run & charge trait all make your otherwise slow infantry get into combat extremely fast. The command ability for +1 to hit is also great.
Lords of the Lodge. Pile-in and attack twice with your 30-man Hzerker unit, obliterate the poor sods you are in melee with.
Runefather on droth (general). Needed for the battalion and to make Hzerkers battleline. Being on a droth makes his 12" bubble for the command trait much larger, it makes him harder to kill, and he is very buffable. Unfortunately an enemy with decent MW output will probably kill him round two or three. Fortunately he is needed most early on for the run & charge buff. Give him the coal-heart ancient mount trait to make him more durable in melee, as well as the vostaxe artifact (since there is nowhere else to put it).
Runemaster. Needed for the battalion, he has the nifty holy seeker ability, and he prays. His base prayer is nice for anti-horde but the reality is you are probably wrecking those already. Still, nice to have. I recommend searing heat as a debuff to enemies with enough offense to damage you (of which there are many at the tourney scene), but prayer of ash can also be good.
Battlesmith. Needed for the battalion and he auto-take for Fyreslayers anyways. You are on point with giving him the Nulsidian Icon, denying spell effects on a 4+ is too strong to leave behind.
Everyone in the battalion is going to run around as a single blob so wholly within 12" abilities can take effect. The Hzekers will delete enemies in melee and the Runefather on Droth will be very durable (except vs mortal wounds) with worsening enemy rend against him and +1 to saves from the battlesmith.
Altogether that is the first 1280 points of your army. For the rest you can further buff that blob and definitely want to improve board control outside of it, plus fill out your battleline.
Runesmiter on foot + 5 Hzekers. These guys camp an objective with the forge terrain piece next to them. Runesmiters always take the forge key. Molten infusion or ember storm are good prayers to have. The former to use on the fyrewall if you bring it (see below), the latter on the Hzerkers if they need to run off & charge something. You need this in the list because a priest is needed for the forge and the Hzerkers are needed for battleline.
5 Auric Hearthguard. They run with the blob, providing missile support that actually does meaningful damage at decent range (unlike the throwing axes), debuff enemy monsters, and can use their sworn protectors ability to keep the battlesmith alive against enemy sniping (and the runemaster as a secondary). Put together the support capacity makes this an essential element to the list.
That brings you to 1640, plus at least 120 for another Hzerker unit for the third battleline bringing it to 1760. So you have 240 points you can flex to what you want. Some things to look at:
-Runic fyrewall. The utility of being able to block movement and line of sight cannot be understated. Re-rolling saves of 1 is an awesome buff to have on your blob.
-Another runesmiter to run with the blob and give them wound re-rolls & another support prayer (searing heat or prayer of ash). Helpful, but the blob is also really powerful already. Alternatively (and probably more useful) he can run with a 10-man Hzerker unit (below) to buff them.
-Another 5-man Hzerker unit or bumping one of the existing ones to 10. This is simply body count and backup for when the 30-man inevitably takes damage. If you go with this you really want another hero to run with them to make sure they get that 4+ beard save.
-Grimwrath or Doomseeker. Can run with a 5 or 10 man Hzerker unit, and can take the vostaxe instead of the runefather (who does not need more offense).
-Fjul-Grimnir and Chosen Axes. A nice self-contained combat character & unit. Bring a 5-man Hzerker unit along.
-Runeson(s). A nice command ability. If you are re-rolling wounds use it instead of the Vostarg hit buff. The javelins are good, since he will die pretty quick in melee and the war-axe isn't very good. Having two of them near each other gives them re-rolls to all hits which is quite potent on the javelins and gives them character-sniping potential even against look out sir.
-5 more auric hearthguard. More shooting support, more guys to eat wounds for your foot characters.
-Runefather on foot. No slouch in melee plus a useful (and stackable) command ability. Also nice debuff potential.
-50/100 points left unspent for CP. Extremely useful if you are using a runeson or runefather on foot. Also helps get that triumph roll if the tourney uses those.