But if your vpn has a zero day, that lets you get to the ssh server. It's two layers of protection, you'd have to have two zero days to get in instead of one.

You could argue it's overkill, but it's clearly more secure

Only if the VPN means you have a VPN and a jump box. If it's "VPN with direct access to several servers and no jump box" there's still only one layer to compromise.