What Jeremy Crawford Says
Jeremy Crawford answered the question of what is running water.
To the question:
... Is this only from natural running water, or any water that move?
A body of running water from any source can harm a vampire.
He then clarified, when asked about water elementals:
A water elemental is a creature, not a body of running water (a stream, a river, a waterfall, or the like).
How it applies
So, based on that, If you can get enough water out of the decanter that it is stream or river like for your DM, then you're good. But even with the clarifications it is a DM call. I personally think 1 foot x 1 foot of moving water is enough to count as a body of running water.
I, personally, would give the vamp a dex save (DC 15??) to see if he can dodge it before it hits him.