Here are a couple of companies in Portland to check out:
Pedal Bike Tours
Portland Bicycle Tours

Each of them offer tours of the Willamette Valley via bike, and seem competitively priced.

Also, if you’re interesting in planning your own bike ride, here’s a good article I found that gives some tips. The first tour is a great loop in an area I know pretty well — and very reasonable for time/distance. You’ll also not be far from some other great wineries: Trisaetum, Colene Clemens, Ayres, and Utopia (though they may be harder to access with dirt roads).

Finally, keep in mind that there is a budding cluster of urban wineries in Portland proper, and you could have a fun day tasting wine right in town. Check out the website for this group of in-the-city producers: http://pdxurbanwineries.com/
Hope you have a great trip.