Yes, because the decanter produces running water
You've demonstrated in your question that it's mechanically possible to force the vampire to end its turn within the geyser produced by the Decanter. That geyser is running water, which means that you can force the vampire to end its turn in running water and take the ensuing damage.
In the remainder of this answer, I'll explain why the geyser is running water (redundant thought that might sound):
- Using 'running water' as understood in common parlance (everyday speech)
- Examining the dictionary definition of 'running' as applied to water
- Explaining that we're not beholden to design intent/unofficial sources
Defining running water
'Running water' isn't a rules term, which means we take its meaning from common parlance. 'Running water' is a very broad term which can be reasonably used to describe many kinds of moving water, such as:
- The running water that comprises a stream
- The running water that comes out of a bathroom tap
- The running water that flows through a gutter
- Any non-stagnant water
In everyday speech, the word coupling 'running water' can serve as shorthand for natural bodies of flowing water such as rivers and streams. However, the broader definition of the term is best understood by examining the effect of the adjective 'running' on the noun 'water'.
Dictionary.com lists several definitions for running, including:
- galloping, racing, moving, or passing rapidly.
- flowing, as a stream
The latter definition can be presumed to be derived from the former; 'running' describes the way that water flows in a stream because 'running' describes something that is 'moving or passing rapidly'. Hence, any water which is moving or passing rapidly can be considered running water.
Ignoring design intent
Rules designer Jeremy Crawford does not agree with this ruling, and has said so in a tweet:
Running body of water =/= body of running water.
Jeremy's tweets are no longer official rules, so that doesn't matter. His interpretation of the rule is that 'running water' should be understood (as it often is) as shorthand for a '(presumably natural?) body of running water'. However the Vampire's stat block makes no mention of a 'body' of water, and only specifies that the water must be running. I've demonstrated above that this phrase has a much broader definition than select natural phenomena.
The '30 foot long, 1 foot wide stream of water' produced by a Decanter of Endless Water is running water. If you force a vampire to end its turn there, it takes 20 acid damage.
A DM might decide, based on their understanding of vampire-lore, their interpretation of 'running water' or their reverence for JC's design ideals, that only natural bodies of water count for the purposes of harming vampires. Rule 0 gives them license to do that: They're choosing a narrow adjudication when the rules support a broad one.