How do I make a power bite attack? I seem to be only able to attack with the left and right claws in beast mode, or the raise dead and life drain in Vampire Lord mode. Can someone please help with this?

Also, why can you not be in first person mode as a vampire?

The power bite attack triggers automatically if you swing at an opponent with about 25% or less of their health left. It is also worth noting that it only works on humanoids and only certain types of humanoids. It usually seems to only trigger so long as you are dealing with an NPC that is made up of one of the races. It won't work on trolls, bears, skeletons, draugr, etc.

I wouldn't recommend relying on it at once you've perked out the entire tree. The bats drain ability will make it very difficult to avoid killing them before the attack hits, that or it seems like the attack itself is completely dependent on the angle from which you use your last attack against the enemy.

As for why it is restricted to first person only, your guess is as good as mine.