Never taught Vader how to do the lightning? He taught it to Dooku. I don’t know how long he was his apprentice, but surely Vader should have learned it by then. You could argue he can’t do it cause he don’t have hands. But his mechanical hands seem to be fine for force choking and force pushing.
Wouldn’t it be too dangerous for him to learn it ? His parts aren’t plastic if I remember it well.
[and yeah it was a huge mistake - among others - to show Dooku mastering the lightning attack…]