is robloxfpsunlocker allowed in the speedruns? or does it have to be completely vanilla

probably isnt

well I use it in runs, but nothing seems out of the wrong about it, like run wise

The only thing it does is raise the max fps, its like mine Minecraft or Fortnite, you can change what the max fps you want to have is, I would say it would be allowed.

It is allowed. It doesn't really give an unfair advantage apart from separating gaming PCs to bad PCs, which is not a situation most speedruns tend to look at. You will do better in your runs with more FPS however we will not go ahead and restrict that, after all we get more accurate speedruns.