85% would still 1-hit a rogue until he managed to get 7 HP. You'd need to take it down to 80% to ensure a level 1 rogue with 5 HP could survive. Of course, you could always just round down instead of up
A vampire's steal shouldn't be able to kill players... Except for one extremely memorable time at the GDC stand when someone managed to get killed by a vampire stealing health while heavily corroded and the extra damage nuked him
This is a bug. We've got it on the list of stuff to fix.