Don't mix framerate with bitrate. 29.970 fps is NTSC standard. You can read something about it here:

I am not mixing them
Youtube asks for 30fps (not necessarily, they just say it's common)
You can record on obs at any fps you want, like 30fps
And sony vegas only allows you 29.970fps

Bitrate is about quality and pixels on the video, the rate of bits that are going in there
FPS are the frames or the images shown on the video, if it's 30fps for example there will be 30fps going in less than a second during the video

It's a legacy of the video world that clashes with new world computer way of doing things. Problem is, we still have TV's, Broadcast, and lots of PROFESSIONAL video equipment that adheres to this standard. So you should too - if you are a video pro; otherwise you can do whatever you want. This kind of confusion also rears its head with levels (ie 16-235 vs 0-255). And it is getting worse as more and more gamers record video from one digital medium to another with no camera involved at all. Oh joy!

YouTube, and pretty often elsewhere, like many camera menus, 30 is 29.970 rounded up. Just upload the 29.97 to YouTube. 30.000fps is pretty rare (although from memory a TX1 I used to have recorded at 30.000 and it caused all kinds of problems with sync and TC).

There is a known issue in the current version of Vegas that prevents you from rendering out at exactly 30fps w/ Magix AVC.

You are correct in thinking that you should be able to manually set it to 30fps but at the moment it will revert to something more common I believe. Hopefully this bug will be fixed in the next update which is due at any time. See: https://www.vegascreativesoftware.info/us/forum/known-issues-in-the-latest-version-of-vegas-pro--108942/

[VP15] The MAGIX AVC/AAC codec does not allow custom frame rates to be typed in, such as 30.000 and 60.000

You could also try using Sony AVC or another codec if you don't mind forgoing the extra speed provided by GPU support (exclusive to Magix codec I think).

In most cases you should render out to the same frame as your source. If you're recording w/ OBS then until this bug is resolved it's probably best to record at 29.97 and render out at 29.97 from Vegas. If you record at 30fps and then render out at 29.97 you may run in to minor audio sync issues & dropped frame issues.

I've never noticed a difference between uploading 29.97fps or 30fps to youtube. It seems to handle both the same. You can see what fps (and codec) youtube is playing back at by right clicking, and picking "Stats For Nerds".

If you have 30fps videos that you need to render out when only 29.97 is available make sure that Resampling is disabled (right click on the clip, properties, and then under Video Event make sure it's set to disable resampling - or project settings but then make sure resampling is disabled in project properties). If you don't do that Vegas may end up inserting soft intermediate frames to try to cover for the minor frame differences instead of just dropping a frame. This usually ends up looking really crappy (ghosting effect). It's probably more of an issue when the source FPS is less than the output FPS. That said with resampling disabled Vegas will just drop a frame whenever necessary which you won't notice.