Hi, when i produce mp4's (sony avc) with Sony Vegas, they play nice and smooth on computers or internet. When i play them on my blurayplayer (as a data-bluray)....
Internet Ready Blu-ray Player | Blu-ray Disc Player | BDP-S185 | Sony USA (http://store.sony.com/p/Internet-Ready-Sony-Blu-ray-Player/en/p/BDPS185#specifications)
The audio delays further and further, thus out of sync.
What could be the cause of this?

Perhaps you inadvertanly put and effect on the audio?

Nope...straightforward audio.