Automatically-reconnecting TCP tunnel

Is the old an unmaintained Reliable Sockets (Rocks) what you're looking for ?

The only standard protocol I know of with this capability is MPTCP. It is transparent to the application layer, so SSH on top of MPTCP should just work. It can run the underlying TCP connections over different paths with different IPs, so in principle it could be used to migrate your SSH connection in and out of the VPN connection depending on whether the VPN connection is up.

I don't know much about the maturity of MPTCP implementations, but the design of the protocol looks quite robust.

It should protect your SSH connections from getting lost due to flaky network connectivity. It won't protect you against a mitm who wants to break your SSH connection. A mitm can still inject corrupted data, which SSH will detect and break the connection.

An MPTCP like reconnect method build into the SSH protocol would be the method I could imagine keeping a connection alive for the longest possible time. But I don't think such a feature has been designed for the SSH protocol.