Santiago15's post is quite interesting and accurate, but I think it may be worth mentioning a few other factors into the mix:
VR is not strictly independent of the runway condition. There is in fact a range of speeds within which the climb gradient requirements can still be met. This means that there may still be scope for a small VR and V2 reduction which would recover some of the tarmac lost while accelerating on one engine from the lower V1.
Also, if the V1 speed must be changed significantly because the runway is now wet, it implies that the runway was weight limiting in the first place. Therefore, a reduced RTOW would allow a recovery of effective distance available. It is a little confusing to compare the dry and wet cases for cases at equal weights. In reality the wet runway would require both a reduced V1 and likely a reduced RTOW also.