openssh is one of the most secure projects. It's developed by the security obsessed (and I mean it in a kind way) folks at OpenBSD.
I, for one, am ready to place for more trust in openssh than in any VPN daemon. The most commonly used ones are propitiatory.
What if there is a 0-day vuln (not exploit) for these VPN daemons? That far more likely. "Securing" ssh with a VPN is just one step beyond of security by obscurity.
If you are afraid of script-kiddies and scanners, let your sshd listen on a non-standard port.
I don't think that exposing SSH to the internet is that bad, but your argument is not sound - requiring a VPN does add security, because if there happened to be a vulnerability in it that allowed access, all it would do is expose SSH on the machines (I'm assuming you have proper firewalls set up), which you are advocating making public in the first place.
Saying it adds no security is false, because you'd require an unpatched vulnerability both in your VPN server and in the SSH server simultaneously. A zero-day in one is possible, but in both at the same time is far, far less likely.
You're not securing ssh with a vpn. You're adding another layer. ssh is still secured by all of ssh's existing protection.
ssh behind a vpn requires that someone both compromise the vpn _and_ compromise the ssh service to gain that access that, without a vpn, would require them to only compromise the ssh service.