Another question would functions be quicker if I declared the arguments as const's where possible?
//A
setVetex(int x, int y, int z);
//B
setVetex(const int x, const int y, const int z);
The const "can" be faster but it depends on the use/case involved. For instance, is the function defined in the header or in a cpp? If it is in the header you can ignore pretty much everything since a simple function such as this will inline given just about any compiler. But, even in this case the const "can" be faster based on usage since the non-const version could require the compiler to make copies of the values. Assume that the setVertex is non-const and it is in tight loop, prior to inlining the function the compiler may decide to make copies of the passed in data instead of just using it for the assignments. "Most" compilers should pinhole optimize away the copies since it will notice the fact that there are no changes being made. Of course depending on the complexity of the code involved and the compiler involved, this "may" not be caught and as such case the const version helps the compiler understand the intention and make sure the optimization of avoiding the copies is applied.
Const is a rule for your code to follow and a hint to the compiler, it really doesn't mean anything in the real code. But it "is" a good hint to include when and wherever you can since not all compilers are equal when it comes to doing the best optimizations possible.
Moving back to the prior comments about the speed in general, this is much more arcane than any simple answer can support. I look at the overall class and "not" any individual function in the class. For instance, what happens if/when you decide to change your vector to use SIMD math. SIMD is faster even in the case of 3 component floats but comes with some downfalls. Worse, some of the suggestions suck when you go to SIMD. Given this is the beginners area, I'll stick to the simple case that you should be as const correct as possible when defining your classes of this type.
I could go on for a while about many things involved in this, but it does not have much place in a beginners area.