Try casting greater malision befor attacking with the vorpal sword in BG2. It makes a world of difference
I personaly think the vorpal weapons should not allow a saving throw, but should deal much less (ie. half) as much damage as there normal counterparts. And only have vorpal hits on 10 - 20 percent of the succesful hits.
If the silver sword had vorpal hits 15 percent of the time, no save, but only dealt 1d4+1 damage on normal strikes, it would be a very formidable weapond indeed.