Darren,
You must look at the job logically.
It is a different kettle of fish between a vice between used at home and one that is used for production.
If you have a nice play free action, and the jaw isn't wobbling up and down when you are tightening it up, leave it at that. Tapping down onto parallels should be enough for what any of us want to do.
I doubt very much whether you will be using enough cutting force to cause problems in that direction, and if you start to try to make it anything other than what it is designed to do, you could end up with a candidate for engine raw stock.
John
