i hear the pacific NW is a bad climate for vinyl tops...but i really would like one on my 68. Its an outside car so im wondering how long one would last and what precautions or prep can i take to avoid bubbling or other serious problems

As long as the roof is prepped properly, it should be OK. If the car needs to be painted, I would strip it, do any bodywork that needs to be done, then primer it with a laquer primer. After that, block it down with 400 grit and if you are happy with it, lay some DuPont 1986 Sealer down on it then paint it. It should last a good long while.