The hunter-killer missiles are really overpriced and unreliable considering you only get one shot with each one. You would be much better served by replacing them with autocannon heavy weapon teams in your infantry squads. Hell, you have so many of the damn things, you could buy a second vendetta!
Also, a single BS3 meltagun from a mounted infantry squad is not particularly reliable, and the short range precludes you using the vehicle as a fire bunker. For 120-points, an infantry squad with grenade launcher & autocannon, riding in a chimera, makes for a great basic unit. 4 S6 shots on the move, adding 2 S7s and possibly 3 S5s if you chose the hull-
HB when stationary = fantastic. You also get the option of combining your squads in pairs or triplets over objectives, so you can order them Bring it Down and use the chimeras as empty speed-bumps.
As far as meltas go, if you take two veteran squad with meltas (and demolitions on one or two), in addition to the manticore and the breacher medusa, you have your heavy anti-tank more than covered. The addition of autocannons above would improve your ability at knocking out transports from range, which is the preferred way to deal with them over meltas (after all, if the marines get to within 12" of your lines, their transports have done their job). To your platoon command squads, I'd give 4x flamers, which is one of the most brutal 50 points of models in the game.
If you make these changes, you will only lose a tiny bit of effectiveness vs AV14 (you can easily deal with 2-3 of them over a turn or two), but significantly improve your ability to deal with transports/
MCs from long-range and with hordes up close.
A few general notes on the
HQ choices: If you're going to give the
CCS carapace armor, the medic is really not necessary, so you'd be better served with another plasma gun and 15 points in the bank. Experience with guard has taught me to always have a vox network even in a mechanized list, but that's a personal choice. Officer in the Fleet is great, but in my experience all too often simply ensures all of my opponent's stuff just rushes in on turn 5 and smacks me around without a chance to strike back before the game ends. Perhaps better deep-strike protection can be derived from an allied Inquisitor, who can also defend you against psykers (ware the Space Wolf and the Blood Angel!) Astropath is a decent choice if you face a lot of drop pod armies, otherwise I wouldn't bother.
And as for your Vendetta plan, remember that you can start with the Veterans embarked at the beginning of the game (since the Vendetta is not a dedicated transport). Then you can make a 24" scout move, and have the option of disembarking, moving and assaulting. This is great fun with a demolitions veteran squad against a pair of aggressively-positioned Land Raiders.