Are you serious? You step on the brakes and the VX keeps rolling? If it's happening on dry pavement it's not a tire problem man - that's a brake problem!

Even if the tires are worn down past the wear bars it should stop just fine on clean, dry pavement. Kind of like having race slicks - without the sticky compound, of course.

I'm sure if it were something blatant like the brake pedal going all the way to the floor you would have figured it out and would have a new master cylinder by now. Must be something more subtle....

Does the brake pedal feel spongy? Maybe you've got some air in the system somehow. No that can't be it because you say it only happens sometimes. Do you live in an area where there's a lot of sand? Maybe it IS the ABS kicking in when you're braking on a sandy patch in the road. You can usually tell when the ABS is activated though...

OR - how about this - does it occur just after you've put Armor-All or something like that on the tires or cleaned the wheels? I'm thinking overspray on the disks here. Would cause bad braking until it burned off...

I don't know - but it sounds dangerous. I'm pretty sure it's not a tire problem though. You can have steel showing at the tire edges and that VX would still stop just fine! But not for long...