ISP DNS Bypass

I am using a RB750. I am running it as a ppptp client. The Lan clients behind the Router are routed to the ppptp server.
The routing is working proper.

My problem: The clients behind the router are getting the Router DNS. I am able to change that, e.g. in
208.67.222.222 which is the DNS from Opendns.

Vista client, ipconfig /all is showing the DNS server from Opendns.

But the Domains will resolved from my local ISP. I need an idea that Domains will resolved from my
ppptp server.

If I use the pptp tunnel with my vista and without the Mikro its working proper.

I was trying many things out, but have still no solution for that.

Thanks in advance.

Cheers
Skiller