Your question doesn't really make sense seeing as 60p or 30p are both usable in vegas in their native forms.
You mention the rate of 0.4 for 60p footage so I will assume what you are trying to do is interpret the footage as 24p to fit on a 24p timeline in. If this is the case, the playback rate will be 0.8, or be slightly less (by a few decimal points) if your source footage is exactly 30p rather than 29.97p.
Yea sorry for not clarifying. Im trying to put a 30fps or 29.97(whatever the t2i does) in a 24frame timeline in vegas and need to know what to set the playback rate at to make it smooth slow mo.
For future reference, you just divide the project framerate by the footage framerate, which in this case would be 24/30 or 23.976/29.97. As stated above by David, these give you a playback rate of 0.800.