Assume positive intent, and look at it from his point of view -- which does not mean skipping vacation or moving it again.
Rather, figure out how work will get done in your absence, and brief him on the plan you've made.
"Boss, thanks again for green-lighting my vacation. I need five
minutes to brief you on my coverage plan for when I'm out on vacation
March X-Y. When is good for you?"
(You've already written, and done the work of, the plan before you say that.)
"Boss, here's the plan. The Smith work is due while I'm gone, so I've
already done most of it, and Becky is ready to finish it off when the
specs come in. The Jones project won't be due for another two weeks
after I return, and Andrew has been briefed and will field any calls
that come in from Jones while I'm away. And the regular pop-up
administrative work will be taken by Joe, Jill and Chris in rotation
-- I've cross trained with each of them.
"That's it. Anything I've overlooked? Any concerns?"
And that's how you remind your boss of your already approved vacation.
If he gives you grief, you can always suggest that, if you're truly irreplaceable, maybe he's hinting he wants you to ask for a raise.
Also if you have access to his calendar (or a shared calendar) just post your vacation days -- and those of other folks, of course.
Stand up for your rights without apology. Just like he would if it were his vacation.