I have the Corsair Vengeance 8GB (CMZ8GX3M2A1600C9) already on my ASUS Z97-A ATX DDR3 2600 LGA 1150. Would getting a Kingston HyperX FURY 8GB (HX318C10FWK2/8) perform better or should i just buy another Corsair Vengeance 8GB (CMZ8GX3M2A1600C9)?
Combining RAM from different brands and type should work but try to match the freq, CL and voltage to be the same.
If the combined RAMs are different, the chance of having issues is high.
The best solution is to use the same brand and type.
As someone who has tried mixing different brands of ram with the same voltage/clock speed/latency and having a hard time trying to get them to be compatible with each other I do not recommend mixing brands.
The best solution would be to get the same stick again as it will be much more likely to work. You will also see no increase in performance by using a faster ram stick as both modules are forced to run at the same speed (the speed of the slowest) for your system to be stable.