I just skimmed through all of the posts in this thread, so forgive me if I go over something that has already been discussed.
I see in one of the posts that you got under the VX and checked the play in the front half shafts, but you didn't say (that I could see) if the CV boots were intact or not. Are/were they? How long have you had the VX and do you know if the CV boots were replaced, or not, before you bought it? The original boots are black, and would probably be a little dried out due to the age of the VX.
Do the popping and grinding sounds "feel" (sound) like they are coming from right under the front seats? Can you actually feel a grinding/vibration in the floor board under your feet?
I ask all of that because those were the same sensations that I was getting when my driver side inner CV outer race was going out. I know that's a confusing statement, but what I am ultimately talking about is what everyone calls the "green cup" on the front axle. If you get under the VX and look at the axle housing, the part that doesn't move and is bolted to the frame, you will see what looks like a big green cylinder sticking out of the axle housing. The inner CV boot should be wrapped around the end of this. That cylinder is actually the outer race for the inner CV joint. My inner CV boot split and I didn't catch it for a while until I started getting some strange grinding noises and vibrations that seemed to be coming from right under my seat when I was driving. It ended up that the green cup had gotten some junk in it and the CV joint was going bad. I think the fact that the axle is hard bolted to the frame the grinding sounds and feelings were traveling through the frame and I felt/heard them under my seat.
That's all I have for now, and I really hope that it isn't your green cup because it's a bit of an involved process to either replace it or rebuild it because it is actually the end of the drive axle that is inside the axle housing.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless