I use Vegas 8 Pro to render, using the following parameters:

My source video is 1920 x 1080i

Main Concept AVC/AAC (*.mp4)
Template Custom 1920 x 1080
Profile Main
Frame Rate 29.970 NTSC
Field Order Upper Field First
Pixel Aspect Ratio 1.000
Number of Reference Frames: 2
Use Deblocking Filter: Selected
Variable Bit Rate, 2 Pass
Maximum 20,000,000
Average 14,000,000

Have done this several times before with different projects, only this time the resulting .mp4 file cannot be played by any of my players (Arcsoft TMT, VLC, Nero, or Windows Media Player). All of them spit out the error message "Unsupported file format".

Any ideas?
Thanks,

It is possible that some other software install has replaced the MC codec with a new one - do you recall installing anything with MP4 capability?

If so, usually, the swiftest repair is to reinstall Vegas.