If one person being incommunicado requires that another "delve too deep", then you need to invest in having the people on the team train each other on how their section of the project works.
If you haven't yet done that, whoever's their backup needs to go ahead and delve as deep as necessary.
Or you need to be willing to tell the customer that the best person to handle their request is out but will be back next Thursday and can they possibly wait. In many cases, the difference between now and Thursday really doesn't matter to the customer. A SEV3 bug is not a SEV1 HOT crisis.
Or, if it's really business-critical, you need to talk with the employees about whether they'd be willing to give you cell phone numbers so they can consult -- and possibly carry company laptops with them so they can work remotely -- when they're on vacation. This requires that you convince them that you will contact them ONLY if they say in advance that you can do so or if you absolutely must, and that you will make it up to them somehow (issue them additional holidays to make up for the time you're stealing back, bonus, raise to reflect their dedication to the company above and beyond the call of duty...)
BUT.... In this case the employee is only testing Software X? That means X is not yet officially part of your toolkit. That in turn suggests you were getting by before s/he started evaluating it. And that, in turn, suggests that while it might be nice to have, it probably isn't something you should call them back to drive unless it's truly necessary -- both time-critical and not solvable any other way.