Interesting thread, I've never used vibrant specificly but my experience in general with bacteria type products is that you should always start with a much lower dose than manufacturers recommendations to see how your tank reacts. Usually if it's a weekly recommendation I try about an eighth and work up weekly while watching the tank. I always dose bacteria while the lights are on so carbon dioxide doesn't build up unexpectedly and when trying a new product generally run an air stone just in case some extra oxygen is taken up.