Hey Goats.
Not sure about the VX, but I had a VT-II S. I never bothered too much finding out about the stock speakers - quite frankly, they're rubbish!
When I pulled mine out to replace them with something half decent, all I could see was "Made In China" stamped on the back of the magnet. (Oh, I think there was something about them being 4 ohm impedance)
Purely an estimate, I'd think they could handle a maximum of about 10-15 watts (RMS, that is) - as long as you don't mind listening to cone break-up and almost non-existent response above about 10kHz. (the VT didn't have tweeters mounted in the dash corners - VX may be different)
Do yourself a favour - if you have the door trims off at any stage, spend about $50 a pair and get some reasonable 2 way coax units. You'll thank yourself, believe me.
I managed to fit 6.5" speakers in the front doors, but had to do a bit of modification to the mounts (plastic spacers which can be removed from the doors, so not too much of a hassle) but 6" units drop straight in.
For what it's worth anyhow.
Cheerz!