Sure. But without a VPN, anyone who compromises even one of your other hosts is in the same position. It's a lot easier to audit a single-purpose VPN server for possible security issues than it is to audit all the application code running on the rest of your production systems.
And I'd rate the odds of a 0-day being found at higher for most VPN software than for SSH.
I wouldn't. And even if you're right, getting a VPN login still doesn't get you anywhere. You still have to be able to ssh to the rest of the hosts. That's why we do security in layers.
You would ssh to the bastion host, and from there to internal hosts.
With the appropriate ssh config at the client end, the tunneling through the bastion can be scripted away (using the ProxyCommand directive and RSA keys).
Also, different attack surfaces. Two layers of the same security measure (ssh) is, all else equal, not quite as good as two layers involving two different measures (VPN, ssh).
I liked the idea of adding VPN layer to SSH, so would like to get as much advice as possible :-)
OpenVPN is the most compatible with a variety of clients. OpenVPN runs in userspace, so the clients for each OS and mobile platform interoperate well. The downside is, it does require a client program to be installed and configured. It's considered very secure, using SSL. Since it's userspace, moving large amounts of traffic means more context switching and higher cpu usage. Despite that, I've found it to be faster and more stable than the alternatives.
L2TP/IPSec is built in to most clients -- Windows, OS X, mobile. But every implementation is different and it's hard to configure a server to work with all of them. There are also more moving parts -- an IPSec server (openswan, strongswan, or racoon), and L2TP server (openl2tpd, xl2tpd) and a PPP server (pppd). IPSec seems to be a secure protocol but it's very complicated. I tend to distrust complicated security.
Pure IPSec has many of the problems of L2TP/IPSec with the added problem of difficult to configure in Windows and OS X.
PPTP is not performant or very secure. Other than the fact that almost every client supports it, I see no reason to use it for a new VPN.
I also agree that SSH is less likely to have flaws than most VPN software. But on a properly configured bastion host, by-passing the VPN would just put you in a position where you can attack SSH. You would still need to by-pass SSH to access production servers.
You are assuming the VPN host is trusted any more than most people trusts random servers on the internet.