Layered security is good, but you're still using a key to access a resource. Looked at individually, using a key to access a VPN is not more secure than using

Layered security is good, but you're still using a key to access a resource. Looked at individually, using a key to access a VPN is not more secure than using a key to access SSH. The risk isn't a remotely exploitable bug in ssh, but losing your secrets due to some other vulnerability. If someone has access to your SSH key, they almost certainly are able to also access your VPN key - both are saved in your home directory and need to be regularly accessible. If, maybe, your VPN uses two factor authentication, that makes it much harder, but you could also use two factor authentication for SSH, and we're back to where we were before.