Congratulations to your son on having two great choices!
Caliber of instruction will be equal at these two fantastic colleges, which are more similar than different in many ways, mainly in all the qualities shared by top small liberal arts colleges.
They even both have a high degree of choice with few requirements.
Students are similarly talented academically at both colleges, and interested in learning and ideas, and professors are similarly excellent.
Campus life:
Hamilton struck me as having an exceptionally friendly atmosphere and a nice mix of students of different types, as reflected in the architecture. Hamilton does, however, have fraternities and sororities, albeit nonresidential. Whether that is viewed as a positive or negative depends on your son’s perspective.
Vassar has no fraternities or sororities. It has dorms where students spend three years, forming a tight community (then, seniors often live in on-campus college-run apartments). It has an inclusive vibe. It was a formerly all-female college (until 1969) and that still has an impact on the “equal” feel to the campus. Both colleges’ students are predominantly liberal, but Vassar has been a bit more known for protests through the years.
There will be a variety of students at both schools. He will find friends who share the interests that are important to him.
Ultimately, with two completely equal colleges such as these two, you can feel confident that whichever choice your son makes will lead to a wonderful four years. Encourage him to talk out his reasons, and maybe visit both again if he is unsure, and then he can make his choice!