Finally add player bar vertical dimension if necessary (like 25px for Youtube).
Video Encoding is done in blocks of 8 or 16px, depending on the codec used. Decoding is much faster when the target display size fits into this 8 or 16px raster, so the decoder doesn't have to unnecessarily re-scale the video image.
Change Vimeo video dimensions to 512 x 288 (playback controls are superimposed)
Change Youtube video dimensions to 512 x 313 (512 x 288 plus 25px player bar)