Is it impossible to use sys-vpn with whonixWS?

I’ve tried various things but can’t successfully combine Whonix and sys-vpn.
Is this perhaps impossible? Has anyone been able to do it?

It’s impossible. Whonix Workstation is intended to be used with Whonix Gateway.
I correct myself, it’s possible with additional configuration:
Clearnet link:
Connecting to Tor before a VPN

Though I’m not sure if it’ll be possible to do in the future:

But I guess there still would be a workaround that’ll allow it to work like setting anon-gateway qvm-tag to sys-vpn.

I’m a bit confused. The link in the second answer states not to connect anything other than sys-whonix to Whonix. What does this mean exactly?

It’s to prevent the user error when user will set some clearnet net qube e.g. sys-firewall instead of sys-whonix as anon-whonix net qube.
There are links to reddit and Whonix forum in the github issue that shows the issue:

When you use Whonix Workstation you expect all traffic to go through Tor without any clearnet leak, but if you set Whonix Workstation net qube to be a clearnet net qube instead of sys-whonix then it could lead to a leak through clearnet.

In that case, what should I do if I want to use Tor and VPN simultaneously?

You can follow the Whonix documentation from the link in my first post if you want to use VPN inside Whonix Workstation.
But you can also use app in some non-Whonix qube connecting to VPN through Tor in easier way like this:
app-qube (based on debian template) → sys-vpn → sys-whonix → sys-firewall → sys-net
You just need to create sys-vpn qube and set its net qube to sys-whonix and then set app-qube net qube to this sys-vpn.