now from what we've seen so far we will be the only survivor from vault 111, but does that mean the vault is not usable as a building area. Also will we be able to go into the vault.

I mean in reality if the vault is still usable it just makes since to use it.

Judging by what we seen of the settlment system, ya not really gonna be able to use a vault, as for re entering it, we shall se.

Sole survivor implies that the protagonist is the only Vault 111 resident that is still alive. However, English is only as precise as the user intends, so...

No information has been released as to the state of the Vault. It may be lethal to stay there for any length of time. It may be pristine and perfectly usable. It may be in one of the designated areas for building. It may not be in such an area. I suspect we will not find out until release date.

I really think it is going to about the same a Vault 112, not much to it. Another good picture of what I think it will be is EPA's lower level. It was cut from FO2. If you enlarge the picture, you can see the 3 in hibernation.

http://fallout.wikia.com/wiki/Environmental_Protection_Agency?file=EPA_sublevel_7_Blue.png

The whole Vault as Settlement thing has come up before. And really we don't know. Apparently to begin building settlement pieces, you activate a workbench. We MAY be able to move a workbench down there to do things, but that's the make it or break it. If we can't, we're SOL.

If we can't do it in the vanilla game, vault settlement will surely be modded in relatively quickly I'd assume. A lot of people think it will be in due to the Fallotu Shelter game being exactly that, a vault building sim. I'd say it's possible, buit not to the degree of people thinking their Fallout Shelter vault will just carry over lol that won't happen, nor should anyone really want it to

...100 plus rooms, and a 50 water pumps.

Yeah 100 rooms, 25 each of 4 different rooms. That'd be so fun lol