I'm building a customer/vendor facing portal app and need a good UI. Does anyone have a preferred UI library for use with SuiteScript?

I wonder if it's possible to use NetSuite User Interface Framework (UIF) in customer/vendor facing SPA

Depends on your use case. Because I prefer Vue I have used Vuetify & Quasar. Quasar when I had a use case where there was a future potential to install outside of Netsuite and interact via RESTlets.