People keep saying there is something about vault 3 is there some special story in there once ur in there?

Theres a quest that involves that vault and you can also get Vault suits for Sarah in that vault. Make sure to save 1st before entering I've heard that there are glitches in that vault. I've never expericened them when I went in but better to be safe then sorry.

What other quest involves this vault?

One of the NCR quests involving taking down Fiend Leaders.

A Brotherhood quest to retrieve parts for their failing air filtration system as well. If you put on a Khan outfit before entering, you can walk around freely in there by pretending to be a Khan. You can also pass a speech check instead of wearing the outfit. This only works on the Fiends inside the vault, not the ones outside. You can get a Khan outfit without killing any Khans by going to Boulder City and offering to negotiate with the Khans there. Then, go behind the counter where the leader stands and there's a dead Khan in the back room. Take his outfit. You don't have to do the quest associated with Boulder City.

Vault 3 was just a control vault (unlike many others that were actually experiments, ie ALL THE OTHER VAULTS IN THE GAME). The purpose was to stay closed until it was safe to come out, and then proceed to repopulate and rebuild. They were forced to open up the Vault after flooding occurred, and began to trade with the locals. Unfortunately for them, they ended up meeting the Fiends...and the rest you can figure out...

The actual use of the Vault is more or less for quests for other factions. BoS needs the air filtration units, NCR needs Fiends killed, etc.