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Alright-new rule here, designed to go along with Vehicles With Toughness Values.

If you would wound on something other than a 6+ and roll a 6 to wound, your target suffers a Devastating Hit. (Note: This only applies if the wound actually goes through-if you roll a 6 to wound and they make their armor/cover/FNP/whatever roll, they are not affected.)

Roll on the following table, with a modifier equal to their armor save minus your AP, with AP- being treated as AP7. (For instance, a Paladin struck by a Melta Gun would have a +1 modifier to the roll. 2-1=1. A Daemon Prince with War-Forged Armour struck by a Bolter would have a -2 penalty to the roll. 3-5=-2.)

0-: No effects.
1-3: The model can only fire snap shots next turn.
4: The model cannot move and can only fire snap shots next turn. Fliers are instead Vector Locked.
5: One piece of wargear (chosen by the owning player) is destroyed.
6: The model cannot move for the rest of the game. If the model is part of a squad, the squad can leave the crippled model behind. Fliers are grounded (Crash and Burn! or S9 AP2 hit as appropiate) on a 1-3, or Vector Locked on a 4-6.
7: The model suffers an extra d3 Wounds or Hull Points with no saves allowed.
8+: The model is removed from play. In addition, Vehicles Explode as per the normal rules. Non-Vehicle models removed by an 8+ result instead have the following occur.

Overkill Shot
All models within d3 inches suffer a wound at half the Strength and AP+2 of the attack.

For instance, if a Scout is killed by a Lascannon and a 6 is rolled on the Devastating Blow table, all other models within d3 inches would suffer a S5 AP4 hit.

Seems like a reasonable way to go. I feel that the 5 result will generally mean that vehicles only ever lose inconsequential pieces of wargear like the smoke launchers they've already used or that storm bolter you never remembered to fire anyway. Aside from that, the chart looks pretty reasonable to me.

I'm not sure I'm a fan of the Overkill Shot rule. A lascannon making a tank explode won't necessarily mean that the lascannon somehow shed some of its own energy (and thus strength) onto nearby models. It just means that the lascannon hit something critical thus causing a reaction that resulted in an explosion. Similarly, the strength of an autocannon that blows up a dark eldar raider probably shouldn't have much of an effect on how strong the raider's explosion is. D3" also seems like a strange distance to randomize. If you wanted to keep it, why not make it a flat 2" so that people don't have to take the time to roll and worry over a 1" difference? I'm not really sure this rule needs to exist. Why not stick to the existing Explodes! rule?

Now all that said, I'm not sure that we really need a random damage table for vehicles. It made sense back in the day when vehicles were rarer and more of a big deal. They were sort of like "mini-bosses" that you were trying to take down with a smaller army, so that minutia was interesting and fun. These days, keeping track of a shaken-equivalent result is more minorly annoying than dramatic.

Rather than a chart, why not simply have devastating blows do extra HP worth of damage? A single extra HP doesn't seem unreasonable. Or, if you really want to keep the "battle damage" thing going, maybe remove the randomness? Instead of a random chart, just say that, upon suffering a devastating blow, the controlling player can choose from a list of debuffs. For instance...
* Weapon Destroyed- permanently lose access to one weapon.
* Immobilized- You cannot move for the rest of the game, nor may you jink.
* Crew Stunned- The vehicle may not move and may only snap shoot. until the end of the vehicle's next turn.

So you have two permanent effects that will let you fire some guns normally or move normally, and you have one temporary effect that makes all your shooting and movement terrible/non-existent for a single turn. It shows battle damage, it's easy to keep track of, and it saves you the trouble of consulting yet another table.

As an optional addition to that last part, you could have a rule like Tank Hunter (I haven't read your other post yet) that allows the attacking player to choose the damage result rather than the controlling player.

Overkill shot is not for vehicles-vehicles explode as per normal.

Overkill is for regular models.

On phone, so will post more detail later.