Bootstrap is the more common of the two in our index: it runs on 9,427 websites against 20 for Vant, and 4 sites use both. Counts come from real scans, not vendor claims.
Bootstrap is a free and open-source CSS framework directed at responsive, mobile-first front-end web development. It contains CSS and JavaScript-based design templates for typography, forms, buttons, navigation, and other interface components.
Vant is a lightweight, customisable Vue UI library for mobile web apps.
4 sites run both Bootstrap and Vant. That is 20% of the sites using Vant, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.