I just wrote a nice detailed answer but somehow I deleted the draft. To save time, the simple explanation (EDIT: I haven't gotten this from experience; I'm just going off what people have told me in the past):
Changing your frame rate to a fixed one produces exactly the same quality as using VFR (allowing the adjust of frame rate).
Checking the allow source to adjust frame rate button varies the frame rate in the rendered video for each source file. That means that if one of your source videos is 30fps and the next one 60, the frame rate will change when the next source video plays (from 30 to 60 in this example). I cannot think of a disadvantage of this, although re-editing the rendered video may be a bit weird (as shown in OSFirstTimer's YouTube video). Also shown in OSFirstTimer's video is that most phones record VFR videos but VEGAS Pro usually corrects that.
If you want to enable solid frame rate (Constant frame rate or CFR) follow this simple 2-step procedure:
Find the source video with the highest frame rate
Set your project to that frame rate
I'm new to Stack Exchange, so hope this helps!
@Gyan in the comments says that this is not the case because "outputting a 30 or 60 fps project timeline consisting of a 30 and 60 fps media clip with 30 or 60 fps output frame rate all result in CFR output."
We all learn something new every day! :)
Also I haven't had a dilemma between CFR and VFR before, I'm just going off what I've seen happen to other people!