I made a character with high intelligence, perception, charisma, thinking there would be a lot of interesting dialogue, but so far it's only cave rats and scorpions, and they kill me every time. Reached Vault 15 but never found any NPCs that you can interact with other than one random vendor in the desert who didn't have much dialogue (although I did find a random encounter wrecked UFO with a plasma pistol).
The land is desolate, save for a few settlements. It's also the case that you likely walked through Shady Sands on the way to Vault 15; and it's certainly the case that if you missed SS, you'll need to return before you delve deeper into Vault 15.
The combat is tough, but it's fair ~and they can run out of ammo too.
* There are enemies that can wipe you out though... so while you
can walk anywhere, you probably shouldn't just yet. I'd say stick to the places you hear mentioned in the game when you talk to NPC's.
** There is also a "Tell me about" button in dialog; on principle NPCs they usually have additional comments that you'll never hear unless you ask. Ask by using names they've mentioned, or names that seem likely for them to know. For instance... If they mention a town by name, you could ask them about it in the "Tell Me About" dialog box, and they might tell you a bit more about it.
Spoiler It's a spoiler...
Spoiler That random encounter was one of the best pistols in the game. http://i271.photobucket.com/albums/jj125/Gizmojunk/zapper.gif