only openvpn can connect to openvpn
you did say "openvpn client" which makes me think you may be talking about the "openvpn client" which is linked to from
http://www.openvpn.net
in reality, that is only for access server
The open source openvpn is at
http://www.openvpn.net/download
that version will not require a ca.crt and should solve your problem
8-)
also, you mean that you use client-cert-not-required