This is a problem experienced in the US as they found the efficacy of vaping unsatisfactory when the bees were tightly clustered. Makes sense to me when you consider how a swarm can survive in the rain as it runs off the bees in the outer shell. I don't vape at all as you well know, but it something to consider if I were to vape. I would vape on a flying day when they are broodless. The efficacy of vaping seems less than optimum on the forum thereof repeat treatments and questions about why that is.