I've been thinking about this idea for a while, and have decided that I don't like it. Some of the problems are in the extreme power swing from MC to vehicles. I don't deny that they should be better balanced, but I think those changes would be overwhelming.
The other problem is that the rules for them are too complicated. In all wargames, you have to find a balance between realism and playability (generally from complicated rules). One always comes at the expense of the other. In this case, we gain quite a bit of realism, at the expense of a lot of playability (through very complicated rules).
My counterproposal is two-parts. A change to weapons to deal with multi-wound models, and a change to the vehicle damage tables. Both of them add some realism at the expense of playability (also through more complicated rules). These proposed changes address what I see as the problems: 1) Multiwound critters are slightly too durable, while 2) vehicles are _too_ vulnerable under 4th Edition.
First: doing more than one wound to multi-wound models. I like Yakface's idea of the critical hit, but it seems too complicated. My suggestion is much more basic. Any model wounded by a ranged weapon that is heavy (instead of pistol, rapid fire, or assault) and counts as a main weapon for a vehicle (strength 7 or better) automatically inflicts two wounds for each unsaved wound (in the manner of C:Tyranid Implant Attack) to multiwound models.
Second: making vehicles more durable in comparison with similarly costed MC. I don't like going away from AV and damage tables because all of the points are more or less "balanced" around them. Instead of changing vehicle stats, change the glancing and penetrating tables to be based on 2d6, to look something like the ones I've kludged in below. Ordnance weapons roll on the penetration table but add +1 to both dice. Rolls to damage open topped vehicles add +1 to both dice for either table. In no case can you go above a roll of a 6 on the glancing table or a roll of 7 for the penetration table. The holofield forces the attacker to roll 3d6 and use the two smallest. Venerable dreadnaughts can force a reroll of one of the 2d6.
Glancing Table:
d6 \ d6 1 2 3 4 5 6
1 shaken shaken shaken shaken shaken stun
2 shaken shaken shaken stun stun stun
3 shaken shaken stun stun wpn dst wpn dst
4 shaken stun stun wpn dst immob immob
5 shaken stun wpn dst immob immob destroyed
6 stun stun wpn dst immob destroyed destroyed
Penetration Table:
d6 \ d6 1 2 3 4 5 6 7
1 shaken shaken shaken shaken stun stun wpn dst
2 shaken shaken stun stun wpn dst wpn dst immob
3 shaken stun stun stun wpn dst immob destroyed
4 shaken stun stun wpn dst immob destroyed destroyed
5 stun wpn dst wpn dst immob destroyed destroyed anh
6 stun wpn dst immob destroyed destroyed anh anh
7 wpn dst immob destroyed destroyed anh anh anh
shaken = Crew Shaken, stun = Crew Stunned, wpn dst = Weapon Destroyed, immob = Vehicle Immobilized, destroyed = Vehicle Destroyed, anh = Vehicle Annihilated
Note: please forgive my poor format-fu. I knew it would be bad, but didn't realize it would be this bad.