It is probably possible, but the vacuum portion of the pump is between the fuel portion and the camshaft.
I would think you would get fuel into the vacuum pump area before it leaked into the cam area, and thus into the oil.
Are you seeing any oil at your vacuum pump exhaust?
This is based on many assumptions; I have not had one apart, and have seen no detailed drawings of the tandem pump. There are many possible different designs, and all would have different failure modes.
Hopefully someone will speak up that has had one apart.