The problem is that Sony Vegas puts the meta data at the end, requiring your entire video to download before playing. It’s not JWplayer.
However, there are several FREE easy fixes. The one that I use is: Meta Data Mover.
Just google that and it’s the first result. It literally takes a second to run and your meta data is at the beginning of the video, allowing it to play instantly. You run it after you’ve rendered the .mp4 file.
Just to note, I don’t believe that the .mp4 method from vegas puts in h.264. What I do for that is render in .avi uncompressed or even in .wmv in high quality and then run it through handbrake (free) to get a h.264 mp4 file with the meta data at the beginning.