It's industry standard and built into all modern systems, nothing personal or effortful about responsive design whatsoever. It could also significantly improve the SEO for the site -- and while the site is meant to be a community hub, it is also most assuredly a business.
vBulletin just doesn't accommodate it by default. You'd want something like Flarum or Simple Forum. Alternatively, you could switch to a responsive VBulletin template like Quarto:
https://themeforest.net/category/for...lue=responsive