Talk to a recruiter, but I think you'll have an uphill battle.
-I tried to go from IRR to VTU, and the recruiter wouldn't do it; I ended up going SELRES for a few years.
-For SWO at least, SELRES is filling up within a few months of the FY. From the CTO: "As of 12 November 2015, 148 of 175 FY16 billets are filled or in-processing"
-I'd imagine other URLs are similar, but I do believe that active duty SWOs were encouraged to affiliate with the VTU until the following FY (when it was full).
From my experience: You can be in the VTU, and either (a) drill with the VTU, or (b) get assigned to a unit (Wink, that's been the preferred thing for a while I believe). If you're assigned to a unit, you'll do everything a regular unit member in your position would do...but not get paid (including the off-DWE stuff). If you're assigned to the VTU, you'll spend your time tracking down all the dirtbags who have been assigned to the VTU because they're medically broken, have gone UA and don't care about the Navy, have stabbed people..whatever. All these people get kicked to the VTU for various reasons, and then suddenly are on the VTU's "hit lists" for flu shots, security clearances, or whatever. I did some time assigned to the VTU, and each DWE was a ton of work...but I didn't have to do anything in between. The next CO offered to let me flex drill or whatever, but I was done at that point.