I think that your son has narrowed down his ED options to two outstanding schools.
OP wrote:
Her son is “…incredibly smart, but very quirky. He needs peers that are bright like him, but also kind and supportive.” “His social skills are not great, and he desparately wants to find a friend group in college.” “Not interested in partying…”.
Between Vassar College and Williams College, Vassar is the better choice for your son.