As i know VRB and PRB is just a term to differentiate the RB in MAC and PHY. Like in Linux we have virtual address and physical address , only thing in Linux virtual address can map to any of physical address in RAM ,the same way here one VRB can map to any one of available PRB. If LTE has 25 Rb's then there will be 25 VRB and 25 PRB. If mac scheduler 5 RB to one UE then those 5 VRb can be mapped direct to PRB or based on RAT(resource allocate type) frequency domain scheduler will map. say e.g if RAT is 2 then direct mapping between VRB and PRB ,will happen like 1 - 1,2-2 .3-3.4-4,5-5. If RAT is bit mapped then it can be 1-1,2-4,3-5,4-6,5-8.