Stefan,
Somethings seems off here
whats the name of your vif, for example vif10g
$int = get-nanetinterface vif10g
$int | gm (gives you all the properties)
But you should be able to do something as simple as
get-nanetinterface vif10g | Set-nanetinterface vif10g -mtusize 9000 -confirm -verbose