Pin vise

I am using a sliding pin vise for wire wrapping. The vise is a mild
steel, but it wears very rapidly against 21 gauge copper, sterling and
gold filled square wire. After about an hour's use, the vise will not
hold wire for twisting. It's not that I have one bad vise - I teach a
class and we have gone through about 15 to 20 of these things. Can
anyone here tell me what's happening and either how to prevent or
repair the damage? My backup advisors are very knowledgible, but we
just can't get a grip on this problem )-: .

Yes, I can use a normal pin vise with a screw nut compressing a
collet. However, it takes both hands and a longer time to grip the
wire.

Charlie