What determines the relevant version of OpenSSL in a given network connection?
Having done further research, I see the following appears to answer my questions:


OpenSSL is just one of many SSL/TLS implementations
OS X does not rely solely on OpenSSL (though the deprecated version of same is still installed with the OS), but rather its own implementation called "Secure Transport"
Depending on the application in use (Firefox, iTunes, Mail, etc.), either OpenSSL (firefox), Secure Transport (iTunes, Mail), or yet some other SSL/TLS implementation may be utilised for the secure handshake
Because the VPN adds just one 'layer' of security, independent of the security implemented by the application, the TLS implementation at the VPN layer in no way affects that used at the application layer (a useful analogy is the layered 'tubes' analogy provided here: 
https://security.stackexchange.com/questions/27891/how-do-a-vpns-route-ssl-traffic-and-do-they-compromise-security