I've located joined all factions apart from the Institute and I've been trying hard to locate them but no luck, anyone can give me a minor hint please?

You have to follow the main story, that's the only way to get to the Institute.

I hoping to avoid the main story for as long as possible after saving Nick at the vault, do you meet them in the next mission?

You have quite a long way before meeting them. Not until you get up to the part where you obtain schematics for a construct that will get you inside.

Okay thanks both, guess I can work on other exploration for a while and maybe build up Sanctuary