It may be (i'm not a vpn expert) that the VPN they use only admits authorised IP addresses, in which case you are indeed stuck. Do you have remote offices that connect via the VPN - they would probably have fixed IP's.
If you can't get around the fixed IP, there are afaik niche ADSL providers that will give you a fixed IP for a reasonable charge - check out
http://www.adslguide.org.uk/