So, we know Shady Sands was Vault 15 people.

Tandi, or maybe her dad, tells the VD that a bunch of Vault 15 people had left with the "best equipment".

So, where did that group go? I took it that there was an argument of some sort, and a group split out, leaving the others, who then left and formed SS.

So, this other group..if bolted with best equipment could be a GECK or two. Unless I missed something or misinterpreted, it seems like an unanswered question.

The rest formed some of the old raider gangs including the Great Khans

Hmm really? Is that in Bible? Or where?

So, that means a lot of the WC raiders were Vault Dwellers in origin?

I'm pretty sure it's mentioned in Fallout 2. They Vault 15 Dwellers formed the Jackels, Vipers and Khans. Khans for definate, others in the Bible I think.