Noblemen in Westeros seem to never, ever marry nonnobles — that Vale marriage LF brokers is a huge exception, and seen as such. Even very minor lords marry each other. How different are they from medieval Europeans?

Depending on what period of Medieval Europe you’re talking about, that’s pretty standard – I guess you could point to the Medici marrying into the House of Valois with Catherine de Medici as a counter-example, but that’s complicated by the fact that a lot of Italian nobility were “in trade” (sure the Medici were bankers, but they were also Dukes of Urbino and  and Northern European nobility tended to overlook such things when it came to Italian real estate, especially when your uncle’s the Pope.