Legends Mode can provide a lot of information. I generally export the info and look at it with Legends Viewer. The Legends Mode info should also provide you with the coordinates of the vault, so it should be possible to embark on top of it (which I think requires the DFHack Embark Anywhere plugin to be set to allow embark on top of it). Once embarked you can use the DFHack command "reveal all" to show everything (except the basement), which should include the vault dwellers (they may be visible even without using reveal).
Obviously, a copy of save should be used for this, as I assume you don't actually want an embark there. Also, I think Legends Mode doesn't provide the mid level tile (i.e. the tile within the embark region) where the vault is located. It's possible to use DFHack to extract the location, but if DF prohibits embarkation on top of vaults you should be able to sweep over the region to see where the 'e'mbark key is greyed out.
Getting the position from Legends Mode should presumably also allow an adventurer to go there, although I don't know much about adventure mode.
Using DFHack, it might be possible to extract the creature description by locating it in the hist fig list, but I don't know how messy it is to get a description out of that. This should be possible to do pre embark, i.e. on the view where you select where to embark, but without actually embarking. Exiting without embarking (but after having used DFHack to get info) you can just back out without any changes to the save (but I recommend backing up the save anyway, if for no other reason that to protect against fat fingering an embarkation).