@VolanD hi there, I didn't find the command to force vlanif keeps up state when none interface is up, but you can achieve this by indirect way,
by the way, you can force the vlanif up for as most as 20 seconds by configure damping time .
first, configure a new vlan which will NOT be used in the furture,
then,configure a new eth-trunk, and configure this eth-trunk port as layer 2 trunk interface, and allow ONLY the new vlan which configured previous and the vlan which the vlanif port is forced up passing.
third, in the uplink eth-trunk port, which is a layer2 trunk port, allows the new vlan passing,
then the vlanif will be forced up.