Just confirming that my setup was somehow possessed. Upon reinstallation, I got everything working as it should. ENB, GPU CP, NVInspector, and iPresentInterval each handle their own Vsync, just like they should. Toggling on-and-off works fine. Something for knowledge on S.T.E.P. overall: A tweak that worked wonders for my GTX 980 ti was to set the following in "enblocal.ini" [MEMORY] ReservedMemorySizeMb=64 Down from the default value of 256. Seems like modern GPUs want to process smaller chunks in rapid succession, rather than one large chunk very quickly. Makes sense, really, to cope with multi-threading. Also explains why these cards run so dang hot. I'll continue fiddling with it to see how low I can get before hitting issues. I know that tweak has nothing to do spINI, just passing on what I've found. (I'm assuming most people will come here looking to make the game run "more better"...so every little bit helps, ya?)