I tried both Mullvad and Mozilla VPN and when I do a dns test, both are still using my ISP’s DNS instead of the VPN’s. This only happens on my Arch systems, works fine on my phone.

EDIT: Turns out these VPN clients depend on systemd-resolved in order to change your DNS. Enabling the service makes it work properly. A bit scary that they don’t give you a warning that you’re leaking DNS if you don’t have systemd-resolved enabled.