Hi Ross,
You can do this in several ways:
1. If your proxy is to be configured on the computer browsers (like ISA proxy), then simply add the traffic from the PCs to the IPs of the proxy to the VPN ACL and to the nonat (with deny).
2. Add all traffic over VPN from the user subnet. At this time you can remove the NAT commands all together since no NAT is required anymore. You can use this even if the proxy is something like Websense that works by sniffing the traffic.
Please rate if this helped.
Regards,
Daniel