Feeding someone hallucinogens against their will (or without their knowledge) is pretty bad news. He could have adverse psychological reactions in addition to illness, and obviously hallucinating can pose it's own risks to one's health and well-being.
That said, Vic was likely more concerned about Branch acting recklessly, going overboard with the vigilanteism and lying to her (and Walt) about what he was up to. Oh, and the downward hygiene spiral, too!