I would consider logic to move all the visual information of the current VFR frame into the current CBR one, if the VFR-TC did not get over half of the CBR-framerate, or the next VFR frame if over. In case a VFR frame is shorter than a CBR one, it should be maintained nevertheless. But I assume this case is rather rare, VFR appeared to save space due to longer frame durations, not shorter.