I've never used Parallels, so here is only a very high level description how you could do it, in case the bridge doesn't work:
- Make sure the Windows system can reach the VPN portal (in your case this probably means: Windows has Internet access)
- Make sure the Windows system has a private IP address on a virtual network adapter, e.g.
vmnet0 that you can
ping from OS X. It's okay if it's the same virtual network adapter that is used by Windows to access the Internet.
- Inside Windows, create a Gateway/NAT. Here's how: http://technet.microsoft.com/en-us/library/dd469812.aspx (ignore the DHCP part, that's overkill)
Now you have all ingredients to reach the VPN from OS X. So imagine this scenario: the VPN allows you to access some private IPs, say
172.16.123.0/24. You need to tell how OS X can reach
route -n add 172.16.123.0/24 192.168.12.34
Read here how to make that permanent: http://blog.irrashai.com/blog/2009/03/how-to-add-static-route-in-mac-os-x/
To troubleshoot and work through this
ifconfig (not to change anything, just read the output),
ping are your friends. ;)