Don't worry about it, I got like a 26 on the diagnostic and ended up with a 40T. For one (and they won't tell you this), I'm not totally sure it's not a little bit rigged so you always do better on you next test than you did on the diagnostic (creating the illusion of improvement). Obviously you'll need to get that VR score up, but knowing your weakness is a great asset; you'll be able to study much more efficiently.
And, as above, practice, practice, practice. There are some metaskills involved, especially in the VR. Once you start to learn how the MCAT works, you'll start intuiting better answers for VR. Good luck!