How to use Whonix over VPN?

Let’s say I have a sys-vpn that when used as a network qube, provides me with a VPN.

I want to do this:
sys-vpn -> sys-whonix -> Internet

What is the easiest way to get this?

You just have to set sys-whonix as the netvm of sys-vpn, be sure to use a VPN protocol using TCP because sys-whonix (tor) only supports TCP.

Wouldn’t that result in tor -> vpn -> website instead of vpn -> tor -> website?
Also, if I want sys-vpn also as VPN-only qube without tor for some other appvms, do I have to keep 2 different sys-vpns with different network qubes configured in each?

No. It would have the VPN traffic running over Tor, which appeared to be
what you wanted.
If you want Tor over VPN then attach qubes to sys-whonix and set the
netvm of sys-whonix to sys-vpn

The Tor Project does not recommend combining Tor and VPN - It’s in their
FAQ

I never presume to speak for the Qubes team. When I comment in the Forum I speak for myself.