Cannot change DNS VPN Properties in VISTA

Hi..

In Windows Vista, I setup a VPN Connection, but when I try to add a dns
Suffix, the VPN Connection never maintain the value and change back to the
default value.
What I do:
- Once I create the VPN Connection, it works fine, but I can't go to
specific devices inside VPN, due to a missing DNS Info.
- So I have to add this DNS Suffix doing this procedure: VPN Connection
Properties --> Networking TAB --> TCP/IP Properties --> Advanced Button -->
DNS TAB --> Here I should change from "Append Primary and Connection
Specific DNS Suffix" to "Append these DNS Suffixes (in order)" --> Add
"mydnsname.com" and Save the changes.
- Until the las step every thing is fine, but when I come back to the same
place (VPN Connection Properties --> Networking TAB --> TCP/IP
Properties --> Advanced Button --> DNS TAB) the default option is selected
again (Append Primary and Connection Specific DNS Suffix).
- I try with different solutions no one works (including Group Policy
Edit).

Anybody knows what is the problem??
and How can I solve it?

thanks in advance for any help..

regards

Luis