smiling larry and the scribe specificly are random events and i want to know if you miss them or something can they respawn again at another random spawn location ?

What defines vendors having levels?

Yes. You can meet them again even in the same location. But it can take a while. Or you need to use save/load method.

The Scribe seems to be fixed. Smiling Larry is still a bit bugged. You can encounter him in the wastes even after making him your settler. You don`t have to hire him again though, he will just go back to your settlement. But it`s a bug.

If you assign them to level 3 store, they have better selection than usual settlers, more caps and also sell some legendary items. So it`s like having level 4 store (=better store).