Yeah, Viper’s TK BK is trickier than the Cammy motion.
The timing is slightly different depending if you are aiming at a super jump one or a regular one.
The regular one, you need to hold down a little longer before doing the motion and then press the kick button as soon as you finish your motion.
But for the super jump one, you need to do your motion fast so that it gives you a super jump then wait something that feels like 3 frames for the super jump startup to actually occur then press your kick button.
If you press the kick too soon, you’ll simply get a super jump.
If you press the kick too late, you’ll get a super jump with a simple HighKick attack.
This needs practice, training mode until you get a good feel of the right timing would be the best thing I could recommend.
(PS: I’m saying 3 frames but that’s just numbers in the air, I don’t know the accurate amount of frames it takes, just wanted to express how fast it feels.)