I need to replace vents in conjunction with siding my house. I have at least one bath fan vent and a dryer vent that run out through the wall between the ceiling in my first floor so I can't get at the connection where they are taped. I'm guessing I will have to sawzall the pipe off flush with the wall but then how can I seal the new connection when I slide the new vent pipe in? Also, I am looking at the dryer vents where the air pressure lifts a cap to let air out, then drops to seal when the air flow ceases, my neighbor has had great results with his. Could I use a unit like this on my bath fan also? My old vents had been hanging an inchor so "open". My 6" HRV inlets and outlets just have a 1/2" wire mesh over them but I'm supposing there's really no way to make these seal when there's no air movement?

Could I use a unit like this on my bath fan also?
no, but you can install a butterfly damper in your bath fan exhaust line. that will stay closed but can be pushed open by the bath fan. here's one supplier:

http://stores.hvacexpress.com/-strse-In-dsh--Line-Dampers-cln-Butterfly-Dampers/Categories.bok