The best way I know to prevent needles from dropping into or out of the cups when hammering a cap on is to preload the cap with grease. It's also a wise move, whether the U-joint is sealed or not. But it's the correct way to pre-grease AND re-grease sealed u-joints, which are stronger & more-durable than greasables.
I also load each cap with grease as I disassemble a u-joint to keep the needles from falling in.
That
Ridgid F50 vise is arguably the best on the market, made in Germany from 70Ksi steel (not common 22Ksi cast crap-Iron) with a lifetime warranty. It's also available larger & smaller (as the F45 & F60).
