I agree with Jay; Do not vacuum it a DSB -and- that’s a lot of volume to surrender (5†in a 28), but I'm just not a fan of DSB's anymore.
Is the intention of the DSB to have a better Biological Filter? How much rock do you have? What kind of other filtration?
Back in the old days, I had a DSB because I had to. These days, technology has changed / improved, to the point where it’s not a requirement, unless your Bio filter + main filtration are sub-standard. (That’s not a dig; I just don’t happen to think it’s necessary, IF state of the art filtration / skimming / live Rock are utilized.)
Personally, I’m a minimalist, and actually prefer having just a modest amount of Live Sand. I presently have a thin layer, not more than 1-1/2 inches (enough for burrowing gobies and whatnot), and I vacuum it occasionally to remove Red Slime colonies, before they bloom. (I also don’t happen to care for bristle worms, which will become prolific and populate a DSB. Of course, that’s part of the solution too, required of a DSB!)
Vacuuming can be a useful way to extract sand, provided you don’t clog the drain tube too quickly, and have a good mix of water along with the sand too. Good luck in whatever you choose. Neither is right or wrong. It’s all personal choice.