When you run ReWire, the transports in the two programs are completely linked. It doesn’t matter in which program you Play, Stop, Fast Forward or Rewind. However, recording (if applicable) is still completely separate in the two applications.
If there is a Loop facility in the synthesizer application, that Loop will be completely linked to the Cycle in Cubase VST. This means that you can move the start and end point for the Loop/Cycle or turn the Loop/Cycle on or off in either program, and this will be reflected in the other.
However, if you are not using the Master Track in Cubase VST, you can adjust the tempo in either program, and this will immediately be reflected in the other.
P If you are using the Master Track in Cubase VST (if Master is activated on the Transport bar), you should not adjust the tempo in the synthesizer application, since that tempo then will not have any effect on playback!