Hi Yugandhar. M,
Yes it is possible that remote vpn users who are connecting to vpn concetrator can access the internal proxy,All depends on what ip address is remote vpn users are getting after sucessfully connecting to vpn.That ip address should be routable in your network and should have access to internal proxy for access internet and as well proper rule need to permitted in your firewall for remote vpn users ip subnets to access internal proxy server.
Hope that clears out your query !!
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Regards
Ganesh.H