There are probably as many opinions on vises as there are woodworkers. The best logic seems to be to get the biggest, strongest, fanciest one you can find and afford. I've read about many people using these Record 53E quick-release vises on the front of their benches as a face vise. Because I found that leg vise, this Record goes on the end. Since the bench isn't hardwood, and the leg vise will be pressing against the bench, I will put a hardwood endcap on the "right" end of the bench made from mahogany; the outside of the Record vise will get a toed-in cheek with dogholes cut from the same mahogany board. This way, the record acts as an end vise for use with dogs on the benchtop, and doubles as a wide-area "face" vise on the end for really tight grips while cutting joinery.
Here you can see the big recessed "mortise" that accomodates the vise's inner face.. I plan on dovetailing this endcap to the two outermost layers to the benchtop, which will get added last. Perhaps this is an overcomplication for my first bench, but then again one could argue that a Roubo is overkill in the first place. I say set realistic goals. If you can achieve them reasonably, without an unjustifiable waste in time and money spent, then the goal is realistic. That being said, pushing the boundaries, accepting a challenge, is at the heart of every maker.
The vise just barely leaves room for the legs.