The Renzetti vices rotate in the hook's axis, but some models only in one direction.
There are lots of vices on the market that are sold as rotary vices. And indeed most of them can rotate in one way or another. But in order to be what I define as true rotary the vice has to live up to certain demands:
1) The axis of rotation has to be the same as or very close to the axis of the hook shank.
This means that when the hook is mounted with the shank horizontally in the jaws of the vice, it will keep the eye and the shank in the same level when the rotating part of the vice is turned. This will on some vices be impossible to obtain for all hook sizes, but will be true for the most commonly used sizes.
Some vices have an adjustment mechanism that will make any hook size rotate 100 percent true.
2) The hook has to be able to turn 360+ degrees without steps.
In order to be useful with some of the techniques and have the advantages that I will later describe, the hook must be able to turn a full round and more. Also there must be no dead points, click stops or anything else that hinders the positioning of the hook. Very small click stops in the order of 5 degrees or so are acceptable, but not desirable.
The very popular Regal vice in the most common position - rotary, but not truly so according to the rules outlined here.
Also the vice must be able to turn more than one round in the same direction.
3) The hook must be able to turn in both directions - clockwise and counterclockwise.
If you really want to use the rotary vice fully, you will need to turn the hook both ways. As we will later see it's not indifferent which way you turn the fly while tying.
Other demands could be that the rotating part of the vice apart from running smoothly without stops or jerks should have a locking mechanism that can lock it in any position. It is also convenient if there is enough space behind the hook for your hands and the materials you want to tie in. Too little space here will make attatchment of tails, tags/buts, hackles, ribbing etc. more cumbersome than needed.
Friction in the rotating part is desirable, unless you use some very special techniques. It takes very little friction to keep the hook steady when you pull on thread or materials, but a totally frictionless rotation - such as those with ball bearings - will probably 'give' in some situations. Ideally the friction is freely adjustable from very loose to completely locked.