Cannot connect to Mac (Big Sur) running SSH server while connected to VPN
If this works when the VPN software is disconnected, then you now have a case to make a configuration change on the VPN server to allow split tunneling.
By default, many VPN servers demand and check that all traffic to the Mac (or PC or Android or iOS or unix client) connect only through the VPN. This causes intentional breakage of ssh since its can no longer listen for an incoming connection from your local subnetwork.
Signing in to a single tunnel VPN intentionally breaks the interface since it exists on the other end of the network tunnel which is a different network range. This is a security issue as most companies consider VPN a firewall to prevent any communications as you have described. This is working as intended for many VPN setups and what you ask may break or be contrary to security setup decision.
Options to change this include:

Try to establish your ssh session before VPN tunnels engage.
Change the nature of your VPN tunneling configuration to allow split tunneling.
Use a network to network VPN tunnel instead of a client to network tunnel.

It’s a good idea to review your security policy if someone else imposes VPN since this could be outside their design parameters or violate security policy if split tunneling is intentionally disabled.