I'm advertising some products on clickbank as an affiliate, and I'm direct linking straight from my PPC ads to my clickbank hop urls. Can clickbank vendors somehow find out what keywords I'm advertising for (so that they can go advertise on those keywords themselves)?
Yep. I can see in my stats when someone comes from a direct or redirect from an ad, it basically shows me the google or other se search, and what the search term was before clicking your ad. Good vendor/aff relationships should stop any vendor from trumping your campaigns. If not - switch vendors and reach out to them directly for open communication in advance.