No way to do this as Epic have used variable scripts which are also hash checked to prevent tampering to enable a stable FPS.
The vista path is:
Your Windows Drive letter usually C:\Users\USERNAME\Documents\My Games\Gears of War for Windows\WarGame\Config
One of those ini files in that folder contains a basic script to adjust FPS but backup these files before tampering with otherwise the only way to play GOW again is a reinstall from scratch as Epic have hash checked all of them so even if you just add a space and save it the game will no longer work at all.
Best way to play GOW is with AA disabled as in places the FPS will drop a lot then make sure Vsync is forced to prevent the terrible screen tearing you otherwise get. Also Vista + DX10 mode is slower and even though it lets you use AA the difference FPS wise is huge so try and run it under DX9 mode in Vista as much faster.