In video transcoding, pre-encoded frames may be arbitrarily dropped to freely adjust the video to meet the network and client requirements. Since transcoding is carried out in real-time, incoming motion vectors are reused to reduce the transcoding latency. In this paper, we propose a new motion vector composition scheme for arbitrarily dropping any frame from incoming video bit-stream comprising I, B and P frames. The transcoded bit-stream retains the I-B-P frame structure. Experimental results are presented and compared to show the efficacy of the proposed scheme.