This one was driving me mad. A file that successfully renders out just couldn't be used to write out an image to Blu-Ray in Sony Vegas Pro 8.0, always failing at the 100% mark with the following error:

Filename: STREAM/00001.m2ts
Status: TSWrapper.dll::CTSWrapper::ProcThreadMain::IO Error --- Failed to open file: 'C:\Users\Ian Smith\AppData\Local\Sony\Vegas Pro\8.0\~bdmux2808\media\......

Turns out that the default setting for burning to Blu-Ray is the cause of the problem (how dumb is it to set a default audio format that won't work??!!!)

The default setting (under Tools -> Burn Disc -> Blu-Ray disc) sets the Audio Format to "Sony Wave64 (*.wv64)"

Change it (using the drop-down) to "AC3" and then everything works fine.

Thought it worth posting here as I'd searched around for a while and not found anything.