Leave your VPN as is. No need to change anything on this point.
I too use Tor over VPN for some apps.
With Orbot in a not VPN mode.
In Orbot you need to activate the Advanced user mode in the Settings.
In Orbot choose applications to route through Tor.
In Molly for example you can check
Settings > Network > Use a Proxy > Tor via Orbot.
If your app doesn't have the option, you can set up the option using the Socks proxy.
In Telegram for example you can check
Settings > Data and Storage > Proxy > Use Proxy > localhost:9050
Hope you will find what you are looking for..
Be careful that Orbot is not recommended by GrapheneOS devs. And is not official from Tor team.