We want to have our framerate as filmic as possible so I'm wondering... What does Vegas do with my PAL HV20's 1080 25p footage in a 24p project? And what does it do with it when I render it out to Cineform 1080 24p? Does it slow it down a frame or does it skip a frame every second?

It will resample each frame, leading to quality loss. The only viable method of getting 24p from 25p has been described by our Bob Grant here:

http://www.sonycreativesoftware.com/forums/ShowMessage.asp?MessageID=551151&Replies=20