Same here, no probs whatsoever with AVCHD.
I have AVCHD from Canon HF10 (M2TS files) and form Panasonic HMC 151 (MTS files). I shoot 1080 50i, 1080 25p,and 720 50p, and Vegas 9c is snappier than ever and performs just magnificent.
I've reported the issue and got the following response after some back-and-forth:
Thank you for reporting this issue and getting back to us with the details of the circumstances under which the symptoms are occurring. This issue has been reproduced and logged into our known issue database, to be resolved in a future update of the application.
I recommend either working with these particular files in the 32 bit version of the Vegas application, or rendering to AVI with the same frame size and characteristics for editing purposes in a 64 bit environment, as a temporary work around until the issue is resolved.