A solid mix that is done "properly" with plenty of headroom should make it through the vinyl pressing stage just fine. Right?
A solid mix that is done properly is all you should ever have to worry about at the mixing stage. The eventual medium on which it will wind up is irrelevant, and its the job solely of the mastering engineer to worry about. But that doesn't mean that the mxing engineer should phone in his own job.
However, my question is if I wanted to have a recording with instruments on one side and vox on the other why is that not exceptable?
I don't know that I can say that it's not acceptable, but it isn't ideal on vinyl. A lot depends upon the nature of the bass (and everything else, FTM.) If the bass is exceptionally heavy and rather attack-y, that can create more problems.
The thing is that because vinyl is a physical medium and bass frequencies cause the largest "bumps" in the groove, excessive bass can really toss the stylus around a bit. This is one of a few reasons why vinyl mastering guys apply RIAA equalization when cutting a vinyl master, it helps keep the bass bumps under control.
When you throw all you bass to one side, instead of the bass grooves tending to just throw the stylus up and down, it wants to push the stylus sideways as well. Too much of this - especially with improper anti-skating on the playback turntable - can make the album more difficult to track without skipping.
As you say, It can be and has been done, but usually you'll find that the bass is not as up-front-thumping. or that the bass overtones might be more accentuated with the fundamentals more subdued, or some other kind of mastering trick(s) applied to make the vinyl more trackable.
G.