I have a customer with a 2910 router with the vpn setup and working. However, in order to connect to their Windows 2003 server I have to manually set the dns address to that of their server.
is there a way of getting this asigned when they get the vpn connection?

I think so, if this is for a dial-in user, you'd need to set the DNS server address on the router itself so it'll always use that - that's done from LAN - General Setup, you'll need to tick the Force DNS option and enter the addresses there, the router will then use those DNS addresses and use those addresses for remote dial-in users as well