Yes. I have found in Vegas, that it seems to utilize all of the frames better if you use a 60p project. The reason that you would want it to do that, is if you are slowing down a 60p shot 40% to 24p. Then when you export at 24p, it will export the slo mo stuff in 24p, and will drop frames in the 60p so that it all (slo mo and non slo mo shots) comes out in 24p.
Someone correct me if I'm wrong.