A lot of folks can get facial flushes from alcohol - rosacea for example.
But getting the same flush from booze that you get from flushing niacin? That seems very strange to me.
That being said, when I have booze the effects of niacin are much stronger. So perhaps the booze (or multivitamin) is activating the niacin already hanging around in your system. I've noticed fair often - if I have niacin before bed, and a multivitamin the next day then I'll notice a very low level flush. I know in theory that should't happen but it does.