I do not see the pump in your picture only a servo However the VP-20 does not have to be mounted horizontal but it must not be inline with the crankshaft, To function it must be perpendicular to the crankshaft so the high frequency torsional vibration can propel the osscillator ball. The Varsane instuctions even illustrate it mounted as one of the possible ways just as your engine is mounted with the backplate mount and the pump vertical under the crankcase.

Also do use the backplate mount as mounting on the firewall or even aft on the enginemount itself will result in depleted pump performance.

John