This question explained how it works, but didn't mention anything about staves. Are staves considered enchanted weapons or do they work differently because they cast a spell rather than causing direct damage?
No, Soul Siphon doesn't work with staves.
Funny enough, using a staff is, for most purposes, "casting a spell" according to the game, except the mana comes from the staff's charge pool instead of your mana pool. Cost reduction effects on you reduce the cost of using staves from that relevant school.