Vaguely recall that there was a check box in earlier VP versions to accomplish your mission?? Would be a welcomed feature.
1. Place your image on the timeline and click the FX button.
2. Click the MASK button - second from the left on the effects toolbar.
3. In the preview window, draw the mask to the dimensions desired and UNtick the MASK INSIDE box.
Click the green + at the top left of the effects toolbar and choose the transparency effect.
Run the slider to zero. (Repeat this if you've altered the mask and a checkerboard isn't appearing in the masked areas.)
Export it as "auto match" resolution. 