I recently discovered that the Google One VPN functionality is available and actually works on GrapheneOS, assuming that you are subscribed to at least the premium/2 TB Google One tier. The Google One VPN is probably one of the better VPNs:
with some background information, links to the source code and a recent audit available here:
Only caveat on GrapheneOS is that the OS' "always on" VPN functionality is not available presently, so you have to make sure the Google One app is up and running after reboot. The app itself will then handle "always on" and also has a failsafe toggle, although both might be less reliable than on stock Android where that functionality seems to be more integrated.
Regardless, the VPN works transparently, almost no latency added and very high throughput. It can be used on ChromeOS too.