well there's quite alot of differences...the biggest being the engine is completely different.vr's had the last of the buick engines used while the vs's had the new ecotec (smaller,lighter and more powerfull).
there's alot of other small changes..but i wont go into listing them all, things like two trans coolers, maf used etc...
visually the vr and vs are almost the same, except for small things like the badges, silver grill insert, slight tweak on the boot lip...
they both have they're strong points...but i beleive the vs having the ecotec is its main attraction.
most of the commodores of that era all have similar problems to look out for when lookin to buy...things like the power steering (leaks, creaking), water/rust in the boot and around the rear windscreen, problems with the central locking and immobiliser, if the car is fitted with rear power windows they most likely dont work from the front buttons, some have problems with the rear main engine seal and sump, poor trans condition..in particular the torque converter (if when held at 80 the whole car shudders and looses a bit of power the converter is gone)....
thats about it for really common stuff..there's always more to look at, just get it checked by a mechanic if in doubt.