Your idea is roughly similar to one I posted here a few weeks ago:
www.dakkadakka.com/Forums/tabid/56/forumid/16/postid/118694/view/topic/Default.aspx
I personally would prefer a system of Toughness/Wounds/Save for vehicles than the current incarnation, but after doing a thorough set of research I am positively convinced that no sort of current work-around can be achieved short of re-writing every codex.
I believe such a change would therefore have to wait for a new edition of the game. I'm planning to present my idea (tidied up a bit) to Jervis Johnson at this year's Adepticon (so any suggestions you'd like to make to my version would be appreciated).
As for comments on your system:
I don't believe that an arbitrary system of applying Toughness/Wounds/Save based on current Armor values of vehicles can be used. Instead, each vehicle needs to be carefully analyzed and given a proper statline.
For example, almost every vehicle has a rear AV of 10, and would therefore only have a 6+ save. Only a very select few vehicles would be any different. That doesn't sound right! IMO, Space Marine armor saves should be better than IG armor saves which should be better than Ork armor saves, just like in the rest of the army. That way, everything just makes more sense to the player.
Also, I wouldn't personally like to use your system because there are even more charts to remember now. Personally I think any re-design of the system should make it even easier to keep track of what is happening to a vehicle (as I believe my version does).
As for what your system is missing? Well, there are literally hundreds of pieces of wargear that have special rules for affecting Armor Value (grenades are just one exampe). You would have to go through each and every piece of Wargear and write new rules. I started trying to do that and realized it would just be too complicated for any other player to ever remember.
But good luck with wherever you're going with this and I like the way you think!