The KB located at
contains an application compatibility update for Vista that include changes for FSX that enable us to fix the disabling of Aero when you launch FSX on Vista.
This came out earlier this year, but I wasn't aware of anything that would make me recommend it. Now I do :--), as that is one of the few issues we are considering fixing in the FSX-DX10 release outside of adding Direct3D10 support. Edit:that doesnt mean we will, just that it is under consideration and this download will be a pre-req.
I highly suggest downloading this update: