VPN and Website Browsing

Hi All

Currently my company is running four different site in which three of these site require a VPN link to our main office. For site A, B, C they remote desktop in to a terminal server. Browsing the net occurs locally through there machines. Broadband traffic goes through one router and VPN through the other.

My question is that we are experiencing problems with certain websites. Internet on each local machine works as normal but for some reason certain sites do not work. We have found that by turning on a VPN dialer on the machine you are using allows these sites to work.

Any ideas as to why this occurs.