I’m running on windows 7 64bit. I am also having this exact same issue. I even get the flickering in the preview screen.
Vegas 9c seem to use a different plugin to render .mts files.
Here is what I get for the render information on the same .mts file on 9c :
Name: compoundplug.dll
Folder: C:\Program Files\Sony\Vegas Pro 9.0\FileIO Plug-Ins\compoundplug
Format: MPEG-2 Transport Stream
Version: Version 1.0 (Build 226) 64-bit
and here is the information on the plugin in 9b with the exact same .mts file. That version is working OK with .mts files.
Name: m2tsplug.dll
Folder: C:\Program Files\Sony\Vegas Pro 9.0\FileIO Plug-Ins\m2tsplug
Format: Sony M2TS
Version: Version 1.0 (Build 2577)
The solution for me was to roll down to 9b. Hope we get a fix soon.