Darren,
I think the hydraulic vices you are on about actually apply more pressure at the final clamping stage.
I have seen them used, but never had the pleasure to use one.
What the chap did was to tighten up as normal, then the final act was to wack the handle with a soft hammer. That brought the internal hydraulics into play that put massive clamping pressure on the part being machined. Much more than can be achieved with normal manual clamping, even by tapping the handle with a hammer.
I don't think that sort of clamping is required in the sort of work we do, unless you are taking off a full width 1" depth of cut.
John