You can hook everything up so that you play the reason synths in reason and vsts in fl, like sanova said.
OR
i prefer to just hook up the midi channels between the two, and them just play the reason synths via midi out channels in FL. easier for me, and i dont have to be switching all the time.
there is a complete guide to this in the FL help thingy. tells you exactly, step by step. if you still can get it pm me or im me and ill walk you through it.
i highly recommend this method of making beats, it has expanded my FL use exponentially...