There have been other threads on the issues with .MTS, so search and see if any of those help.
I would try the following:
Make sure you have the latest/final build of Vegas 12 installed.
Fully patch your windows, so no patches are hanging waiting for reboot.
Make sure your audio and GPU drivers are fully updated.
Use GPU-Z to verify your GPU Bus Interface Speed is operating at 16X speed correct for the card.
Use something like Speccy to make sure that the GPU or CPU is not running to hot during playback/editing.
Looking into converting your source .MTS files into XDCAM, HDCAM-SR-LITE, Cineform.AVI, XAVC-i or use FFMPEG to convert only your audio to PCM. The MTS format is probably AVCHD with an AC-3 sound track(which is really just multi-track MP3.)
Just converting your sound might be enough, and the Pro formats like XDCAM,HDCAM,XAVC, and Cineform use PCM audio.
I generally create a Camera Master folder, and an XDCAM folder in the projects. Then use a new default project to convert the media to XDCAM using one of the batch scripts in Vegas. You can easily swap the media out from the master folder to the XDCAM folder when starting the project.