You can do just about anything, but what to do to get the best quality is the tricky bit. Some people just speed up the 24p to 25p and then convert to 50i. Converting 24p to 50i exactly would require synthesising blended frames which would give you a quality hit.
Nick Hope started a thread on a perfectionist's approach for PAL to NTSC. See if you can find that. It might help.
Yes, with absolute perfection.
Project = 24p
Audio tab, set Resample quality to Best
Put 24p clip on timeline, switch T/L ruler to absolute frames, note number of frames at end of clip.
Change project to 25p, Ctl + drag end of clip until it is exactly the same number of frames long.
Your clip will now playback at 25fps, your can render it out as 25p or add pulldown to make 50i (PAL) by changing simply to 50i.
The above is exactly the same as how any movie is broadcast in PAL land. It will be 4% shorter, no one will notice. The Elastique pitch shifting now used by Vegas is exceptionally good, arguably the best money can buy so even the audio will be perfect.
Note, 24p is not that common, most likely your video is 23.97p, doesn't matter, just match your inital project settings to the media to get it right.
I did the following: I filmed the highway at 24P (as Bob says, it is actually 23.97).and followed the instructions, everything clear to me. Rendered to 1080/ 50i and to 720/25p
Then I compared the original clip w the converted on my PAL TV. The original showed jerky cars, flowing and then kind of stopping and then going on etc.
The converted copies showed no stopping but some "ghosts" appearing now and then behind or before the cars. 25p looked better than 50i. Is this to be expected or maybe id didn't do the conversion carefully enough?
Another clip taken directly at 1080i w the camera was the smoothest.