It has never even happened to me, so all using melee does is deprive me of raising my vampire tree. What is the chance of doing the bite kill-move?

So far in my experience I get the power bite attack whe I use a power attack when the enemy has very low health (so when I know they will die with one more hit)
Otherwise I feel its a waste

Yeah you have to do a power attack with there HP being low (preferably crawling). It can be a pain to do and unlike normal melee its not just an 'increased' chance of happening due to low HP. Also wont happen from normal melee swipes. Low hp - has to be a power attack.

Biggest thing im disapointed in with the form, so far (besides the silly leather back-pants... thing) is the lack of actual melee power attack options. You literally only have 4 'attacks'. Left Swipe, Right Swipe - heavy left or right. Can't do dual combos of any sort, no real diractional stuff I'm remembering. Kinda hope they add that in with a patch.

It's pretty much a kill cam

My VL always get the bite when i even do a slight power attack.

Power bite attack hasn't been very reliable for me. Most of my power attacks just end up killing my target. If you are leveling your perk tree, I would just use the VL drain spell.

Power attack at low health. Easier to do with Bats and poison claws as their health is dropping constantly.

I tried using the melee to level up my guy, but it's not very effective since I got the swarming bats thing. Once you get that, you are essentially stuck using Vampiric Drain to level.