Rulings not Rules
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As D&D 5e doesn't get so detailed with specific damage types, you're going to need your DM to rule on this, or if you're a DM - decide!

* I would certainly reward player ingenuity and allow this effect for one turn. 
* But it would also be possible to make a case for the Vampire not being "in" running water (in the same way it would be if it were in a river). Also the DM could rule that the water canon knocks it back, and out of the way of the water.