According to this, http://fallout.answers.wikia.com/wiki/Where_is_the_sierra_madre_located, it is located SW of New Vegas.

1) What's the source of that information? I can't find that on the Wikia it self.

2) It also doesn't explain what happens after you enter the BoS tunnel and how you actually get there.

Thanks.

Well, Dog/God carried you from the bunker to the casino. At least, that's what was said in the dialogue. Elijah ordered him to do so or something...
As to the first question, IDK. I vaguely recall it mentioned somewhere, but can't remember where.

The Abandoned BoS bunker between the minefield and the river just from Camp Forlorn Hope.

IMHO I think this is the Sierra Madre in the http://gamehdwall.com/wp-content/uploads/2013/04/Fallout-New-Vegas.jpg (to the right, in the mountains) so maybe not too far from the NV area

If, while in the Sierra Madre you place a map marker there, it shows up as west of Jacobstown when you get back to the wasteland--so NW of the Strip. I would imagine it's in California. It's pretty obviously inspired by the Sierra Madre Villa Hotel in Sierra Madre, California.

I always figured it was somewhere south of Vegas.

I think that's just a random casino, since there's clearly alot of buildings in that wallpaper which don't show up on the Strip in-game.