Does CU Boulder or Vassar have a stronger liberal arts feel?

Vassar has the much stronger traditional liberal arts feel. It is a small residential liberal arts college built around broad intellectual exploration, close faculty interaction, and lots of discussion-heavy classes, especially in the humanities and social sciences. CU Boulder can absolutely offer strong work in those areas, but the day-to-day experience is still shaped by being a large public research university with much bigger scale and more variation across departments.

If you want classes that more often center on seminar discussion, reading, writing, and professors who know students well, Vassar is much closer to that environment. The campus culture there tends to attract students who are academically curious across disciplines, and the curriculum is designed to support that kind of breadth rather than pushing students into a narrowly preprofessional mindset. Humanities and social science students are especially visible in the overall identity of the school, not just one part of a much larger institution.

CU Boulder makes more sense for a student who wants some liberal arts opportunities inside a bigger, more expansive setting. You would have access to more majors, more kinds of students, major university resources, and the energy of a flagship campus, but the atmosphere is less uniformly centered on the liberal arts. Intro classes are more likely to be larger, campus life is more diffuse, and the academic culture includes plenty of students whose focus is not humanities or social science driven.

So if what you mean by liberal arts feel is intimate, discussion-based, intellectually broad, and campus-wide rather than department-specific, Vassar is the clearer match. CU Boulder can still work well for a student in those fields, but it will not feel as consistently like a traditional liberal arts college.