if the vox valvetronix amp models are the same as on my korg pedal (and I think most of them are, aren't vox and korg like the same thing?) then I can vouch for saying that yes, they are pretty bad. which is what made me spring for the line 6 of the vox anyway... however, the amp is using a 12AX7 to produce the distortion, so maybe it'll sound better...

btw, yes, a distortion pedal will give you the sound you want, just do as PRS said and run:
Guitar ----> Pedal -----> amp