I'm training this vault right now. Some drills I've done:
-Front layout to land on your back on a resi mat. You can take off from the floor or from a springboard. Then front layout to your stomach (half twist), and roll. Then full twist.
-Block off hands off of a springboard onto a soft mat (otherwise it will hurt). Then same thing plus half twist. Then with a full twist.
-Handspring vault, block to hollow, land on back on a resi behind the vault. Then half twist. I don't know if you'll be able to do a full twist here without twisting early.
-Handspring vault up onto a resi with 8-inch mats stacked on top of it to work the block and body position.