SweetFX does not inject data into the EXE of the game.

While people call the shader mods injectors, they are not code injectors in the original meaning of the word, in that they do not inject code into another process, but have their own separate process.

SweetFX is a set of shaders that run using the InjectSMAA dlls which is a Direct3D proxy dll that redirects the Direct3D render surface to another surface and then runs an extra shader pass on that surface before returning the output to the backbuffer.

VAC rules does not explicitly state that this is allowed, but neither does it state that this is not allowed.

Since Valve have never explicitly stated that it is allowed, it remains a grey area, but in the two years that such shader mods have existed no one have ever been banned for using them.

VAC seems to only detect things which can be used to cheat and shader mods such as FXAA tool and SweetFX can not be used to cheat.

Source : I'm the SweetFX developer.