I installed my custom sub enclosure / amp rack in Aug 2003. See a photo in my gallery here:

If you are building a sealed enclosure like I did, you shouldn't have much trouble damping your rear door. I used BQuiet's Brown Bread damping material, which is better (and a bit more expensive) than Dynamat, but any damping material should help reduce vibrations. With a sealed enclosure, you won't have to worry about covering every square inch of space - only the wide, flat surfaces really vibrate anyway. (Contrary to popular believe, Dynamat and other damping materials do very little to block soundwaves from travelling through, rather, they decrease vibration.) You might also want to consider getting a liscense plate kit - that was the only part of my rear door that rattled - and backing it with BQuiet's VLiner material solved that problem.

As for the hinges, the VX has some pretty heavy duty hinges on that rear door. My enclosure, subs, and amp weigh at least 50-60 pounds, and I've noticed no undue stress in nearly a year. The only thing I've had to be careful of is when parked on a slope (like the parking spot in front of my home) the hinges aren't always strong enough to keep the weight of the door from swinging shut. I've also installed the rear door popper, and while it doesn't always pop the door completely open (I sometimes have to hit it a few times), I believe I can solve that by adjusting the cables a bit. I just haven't got around to it yet.

Sound quality depends on a lot of factors - what type of equipment you are using, how well the enclosure is sealed, volume of the enclosure, etc. The acoustics inside the VX, when properly dampened, are incredible. The thing I love the most about my system is that when all the windows are up and the doors are closed, you can hardly hear anything outside, but inside you are engulfed in clean sound. My only regret is that while my subs hit hard and low, I know that two Orion XTRPro 12's can go even louder and lower, had they been given more volume. With my amp rack and two 12's shoved into that tiny space, I provided the minimum internal volume for them to work properly, but had I gone with a much larger box, those Orions can just put out some incredible bass. If I had to do it over again, I would either make the box larger (so it sticks out of the door more, or eliminate the amp rack), or go with 2 - 10's (which normally aren't capable of the same sound as 2 - 12's, but given the proper internal volume, might sound better than 2 cramped 12's). But all in all, my system still sounds incredible and can play a wide range of music (from Classical to Metal to Techno to Rap) at high volume and virtually no distortion by adjusting only the settings at the head unit. For my first major install on my own, I think it sounds perty damn good.