Well there are exceptions, you are correct. I wouldn't suggest stacking nitrates, in all honesty its just a waste of money. Conquer Unleashed has a very complete profile, you could experiment with adding Vasky but I don't think its necessary.
I don't see any issues with Kill It or C4, but others may chime in with different advice. I've never had any issues stacking Vasky with anything. I personally wasn't a big fan of Vasky, but for the price it isn't too bad. Experiment with what works best for you. Start with a half scoop of Vasky and see how it treats you. Just remember, more is not always better. Check the labels, if your stim-preworkout contains a lot of the ingredients as Vasky does, I wouldn't waste a scoop of Vasky IMO.