Try updating your GPU drivers. If that fails, see if going into preferences and disabling timeline GPU acceleration helps (not ideal, but good for troubleshooting the cause). If that works, we know it is something with the GPU causing the problem. You can then try to disable so4compoundplug.dll to see if that is the cause... so4 is a new decoding plugin Vegas uses that has some issues with certain codecs, so sometimes disabling it and forcing Vegas to use another decoder works. To do this, hold shift while clicking on preferences, then go to the "internal" tab that pops up. Do a search for "so4" and change the second line's "true"'s to "false". Reboot Vegas, and see if that helps.
If all of the above fails, try converting the clip to another format, your cam should come with some conversion utilities.