If you are talking about high vacuum, in the micron range, you will probably want to use copper or stainless steel, very well cleaned.  Same can be said for valves.  Components must be free of any contaminent that can off-gass at the low target pressures.

In the past, I have specified a "truly round" stem with an extra smooth finish on gate valves to be used in vacuum service.  (I can't recall the exact tolerances that we asked for.  The numbers 3-5 micron RA are stuck in my head for some reason.)  This was a client standard that we were following, and there was a large debate within our company on whether or not this finish was really worth the additional cost.  The argument against this was that after the valve had been operated a few times, the packing and stem will wear enough that this extra smooth finish will be destroyed.  If I recall correctly, we also specified that the valves were to be vacuum tested by the manufacturer for leakage through the packing.

I am not aware of any valves that we specified that were required to hold a vacuum in the closed position, so if that's what your looking for, I can't help.  I'd be interested in hearing other peoples insights.

I have no idea whether or not this made any actual difference in the performance of the system.  It is tough to find any real data on how well these valves perform in use.  I do know that these close tolerances and additional testing substantially increased the cost of the valves.

I have seen a few catalogs for valves that can guarantee zero leakage through the use of diaphragms and such, but I imagine that this would only be required if you were looking at a process that would have a reaction with air.  Once again, all my catalogs and data are in a different office, but I know they are available.

Awfully lot of words to say very little here  :)