did you mean that all (3-4) small remote sites is connected to one AE office via ADSL? and this AE office should connect to the SERVER via VPN?
If this is the case, first, you must ensure that these remote sites can connect to the AE office, then, from the AE office, create VPN connection to the SERVER.
Each site, server and AE office, must be connected to a VPN capable device such as a UTM firewall or a router. next, create a vpn connection between the two.
the only concern regarding VPN is that the configuration between the device on both sides should be the same.