My question is: is the vector slowing my program down here, compared to using a simple array? I read somewhere, that the  operator of the vector is defined as inline, and when it's called too often (fe. inside a loop), the compiler doesn't inline it and thus the access is slowed down. Is that right? I'm calling that function from inside a 0..1326 loop.
Vector is *almost* the same performance as a pure array - in release/optimized mode (debug mode at least in visual studio is horrible - vector can slow you down from 1000 to 8 FPS if you are not careful), since there happens almost no range checks, and most calls are inlined/optimized out.
I'm suprised operator is mentioned as being slow - actually,  is faster than at() because  does not perform range checks, at least for map-classes - at map, .at() will check if the index is valid, whereas  will return a valid element eigther way, if the key didn't exist before, it will insert it. I assume it will behave similarily for vector too.
Anyways, for static arrays (int variables), vector is quaranteed to be slower, since it requires at least an additional redirection. Don't use a vector here, eigther stick with pure arrays or use std::array, which is a new std container class for static arrays - vector is dynamic and therefore has certain overhead that doesn't exists for static arrays.