set MTU for adapter RRAS

How to set the MTU size for an VPN adapter configured in RRAS W2K. The microsoft technet article Q120642 describes the MTU setting for an physical interface. The defined on-demand-dial and VPN adapters are generated every time the RRAS service starts. (\Tcpip\Parameters\adapters\) Every thime the have an different .
I want to set an different MTU setting for an specific interface.
Does anyone have tackled this problem before.