lotsocats wrote "Note that when you vacuum up fleas it doesn't kill them. It won't take long after you finish for the fleas to crawl back out of the machine and continue infesting your house!!! The same is true for washing them down the sink while combing the cat. Come back 15 minutes later and you'll find all those fleas you thought you had drowned crawling out of the drain! Ick!"
So, when I flushed the fleas I combed off my cats down the toilet, I wasn't just being paranoid then. I really didn't think they could crawl back up the sink drain, though. I can verify that they do not come back after being flushed.
I used to think bugs vacuumed up would die in the bag, but they don't, as you said. After vacuuming for fleas, the bag has to be thrown out outside.