why the acceleration is reduced only from V1 (or Vef) to Vr, why not beyond Vr and up V2 (35 ft) since one engine is still inoperative?
Perhaps I wasn't being terribly clear in my previous post - I was talking about the effect of V1/VR. You are quite correct that post VR the OEI case is still more critical than the AEO.
Why Vr is the same for AEO and OEI conditions?
We need to have the one VR (for simple minded pilots like me) so it is predicated on the OEI case and generally chosen so that, at the prescribed rotation rate, the aircraft achieves V2 at screen. As RRR observes, this generally gives a modest V2 overspeed for the AEO case .. unless you want an uncomfortably high body angle, especially for twins.