A Vacuum test is meant to ensure there are no leaks.
A hydrotest is meant to ensure structural integrity.
If 100% documented visual inspection was performed on all outside surfaces of the tank during hydrotest, and no leaks were recorded, then you could fell pretty good about skipping the vacuum test.
However, it all depends on the contract documents and the job specific inspection and testing plan. If skipping the vacuum test is not allowed by these documents, then you'll have to approach your customer and ask for a waiver.
You need to point out to your contractor that completing the vacuum test is a tool designed to save him money in the long run. It is much easier and takes a lot of less rework to fix a leak during vacuum testing rather than hydrotesting.

Tim