I don't think motion estimation software is the right solution in this case because, with 240 fps, you already have all the frames you need. (Twixtor, etc. are used to create all those frames from 30 fps source material).

For each event, if you disable resample, set the event playback rate to 0.25 and then add a velocity envelope and set it to 0.5, that will give you 1/8 speed, or 30 fps playback. This will match the 30 fps (29.97) of your project properties and you should get some really, really good slow motion.

I'll be interested to see what it looks like.

[edit] Oops, I just re-read the OP and see that your target frame rate is 23.976 (24p). So set the playback rate to 0.5 and the velocity envelope to 20%. 1/2 * 1/5 = 1/10 so you should get 24p from your 240p source.

I do this all the time with CG I make (not 240fps though!) :)

You need it as individual frames, then you import the frames as a single event, then you specify the frame rate when you import (which would be your target one). You can't change the framerate of video files, only image sequences (would be nice though).

You can't change the framerate of video files, only image sequences (would be nice though).Actually, you can ...

For AVI files, I use this old hack almost every day:

AVIFrate

It works perfectly and is a lot simpler than trying to wrangle things inside Vegas. For NTSC frame rates, you enter 30000 for the numerator and 1001 for the denominator for 29.97. For 23.976, you enter 24000 for the numerator and 1001 for the denominator. For all other "normal" framerates, you simply enter the frame rate in the numerator (240, in the case of the OP's GoPro video) and 1 for the denominator.

I don't know if a similar utility exists for other video formats, but since AVI is a "wrapper," and since there are utilities that can wrap other video formats inside an AVI container, you might be able to run one of those first, and then run AVIFrate.

"I think it uses Vegas' interpolation to do the slo-mo"

No, if you disable resample.
Yes, you can use avifrate, but not with the original mp4 files.

I also like to see the result.
Here is mine, with twixtor, and 720/120 original file:
http://www.relaxvideo.hu/slowmo.mp4

Any tips, what can i do with this strange "hand effect"?
The face looks always good.

GoPro’s cineform version will do it and it’s free.
Import your clip and export it at 24 frames.

Motionperfect as Duncan said will also do it you don’t have to do any motion estimation you can simply export at 24 frames

Actually, you can ...

I was referring to Vegas, but thanks for the link to the other program.

To bad most containers aren't AVI any more.

I haven't tried this entire workflow, but Vegas appears to be happy creating a 120fps progressive project. Perhaps stretch the file by 2X in a 120fps timeline and render an image sequence as an intermediate.

Edit: No preset exists in the dropdown - you need to type in the frame rate in project properties. Anything input larger than 120 fps will result in a 120 fps project. That appears to be the current limit (at least in VP11)

To repeat: the OP's question has nothing whatsoever to do with Twixtor, and that product is not the right answer to his question because he already has 240 frames per second in his camera. Twixtor is used to artificially create 240 frames from only 24 (or 30) original frames per second. All he needs to do is figure out how to keep Vegas from subsampling his 240 fps video in order to conform it to his 24p project.

I never noticed 120 was the limit, but don't forget you can render out faster if the codec supports it.

The Cineform software from GoPro did the trick. Works great!

I'll be shooting some Kung Fu action in the next few weeks and wanted to see what I could capture with the GoPro. Should be fun!