Only 5% of the flea population shows up as adult fleas. So for every 5 fleas you see there are 95 immature stages laying around your house (where the reproduction takes place). Each female flea lays 2,000 eggs in her lifetime. Needless to say good flea control takes more than one application of Frontline Plus.

Frontline Plus takes about 6 hours to kill fleas and they are very active right before they die. Don't panic, they WILL die. You will need to keep treating all the cats every month to gain full control over the flea lifecycle. Vacuuming is a very good help also, but I agree with your vet, don't go the route of fogging/bombing the house. The main ingredient in Frontline Plus is Fipronil and has been used in fruit orchards in the US since the 1980's and only attacks the nervous system of insects NOT mammals. However the chemicals in foggers are much more dangerous, not to mention a real pain to deal with.

Keep vacuuming, keep putting the Frontline Plus on EVERY month on all the cats and eventually you will only see an occasional small flea that has not fed which would have just been brought inside by the outdoor kitty most likely.