Vsync doesn't improve frame times, although it can affect them for better or for worse. What vertical sync does is synchronise your graphics card's output with your monitor's refresh. This prevents tearing, and - in a perfect world - it means that frames are evenly spaced apart.
However, it's not quite so black-and-white. Here are a few neat things about vsync:
Personally, I always play with vertical sync enabled, and I also use triple buffering alongside it whenever possible. I cannot stand screen tearing in native fullscreen modes, and the way that Windows causes non-vsync'd programs to stutter in windowed modes makes my experience less enjoyable.
But you're playing on PC! Play around with settings. The beauty of this platform is that you have choice, and you can tailor your configuration to whatever best meets your needs. If you don't much mind screen tearing, then disabling vertical sync may make things feel a hair smoother. If you prefer an artifact-free experience, then keep vertical sync on and do what you can to maintain a consistent framerate.