Is hydraulic cement a vapor barrier?
No, hydraulic cement is not considered a vapor barrier but it can reduce moisture.
 I have had really good experiances with epoxy coating cement flooring.  The first time I used epoxy I did not expect it to work As a moisture barrier as well as it did. The basement dehumidifier in the winter would remove 1/2 to 1 gallon a day in winter. The next year after coating the floor it was less than 1 quart per week. 
One important thing is to use a coating that can handle moisture if your floor has problems. Tape some plastic to the floor at least a square foot or 3 square foot for a larger test area. After 24 hours if there is water on the plastic or the cement looks wet you will need to use a brand that can handle the moisture or try again in the summer when it may be dry.