I had this problem with 9.0c so I upgraded Vegas to 9.0 d and I still can't work with m2ts files loaded from Sony(!!!!!!!!!!!!!!!!!!!)Xr500V,500,520 camcorders. I tried different drives (including new SATA 6GB/s one) and 32 bit as well as 64 bit versions. When I work with mxf's everything seems to be OK but whenever I start editing m2ts after some time Vegas just freezes displaying "not responding" message. I set up Vegas internal memory to 1024MB. What the heck is going on?!
[I have powerful system - win7 and i7CPU with 12GB of RAM and SLI's so that should be enough.]

I have had no end of problems with 9.0d so did a total uninstall and re-install of 9.0c and no problems since.

i have the same problems with these m2ts. i played with the memory, also up to 1024MB, but still the same.
It freezes by 25% CPU status with around 500MB Memory in use.
But when i select in Preview Monitor a lower sample quality, i have no freezes, This can´t be the solution. :-(