In term of game story
1. Mariposa is broken so the mutant army moving east, which mean from MB to V13 to Shady Sands in term of direct eastward. Or to Junktown/The Hub as southeastern. Or northeastward. It's like a bubble so the group of V13 migrants would be wise to veer wide of it, so that's mean north, northeasterly.
2. Why not Shady Sands you asked? Shady Sands was surrounded by at least three major groups of raiders: The Khans, Vipers, and Jackals. Even if we crush the Khans, there's still two more left. In term of security Shady Sands was not that safe.
3. Why not Shady Sands, you asked? In term of land it's fully occupied by V15 migrants already. And even after 70 years there's no other major town around it. So the original V15 migrants already capture the best (and only) land around.
4. Back to point one, the migrants would travel north, northeasterly. On that direction there's no major agriculture center in the next 70 years: New Reno is a tourist town, and Broken Hills is mining. So instead of continuing inland with further prospect of more drought and less water, they would try to hug the coast line, which there is still no possible great land to farm, even 70 years later.
5. So around the horizontal line of New Reno they would have two direction, continuing northward and later on get to Modoc which is the major farm later. Or they continue seawardly and finally get to Arroyo, which is very good in term of security (that chasm) and existant infrastructure (that big honking stone temple and waterfall)
To be frank, it's like a flip of the coin at that time. Modoc is a better location with Vault City nearby just opened up about 50 years. But well, Vault Dweller must not have gotten any data at all concerning Vault 8 location so he follow the original criteria: safe, easy to defend, and easy to do agriculture, and hugging the sea coast.
As for tribal, tehre's a big article on anthropology here, the which I will spare you and conclude: it's extremely unavoidable considering the size of V13 migrants at that time, and their location of Arroyo. V15 can form medieval village in Shady Sands thank to Junktown and earlier's outgoing group of Khans. V8 has the benefit of several towns around so they form a semi-feudal Vault City with citizens as nobles. But Arroyo has no nearby town (both Klamath and The Den formed much later), so tribal society is the only possible result.