I live in Tigard, Oregon. Should I close off the crawl vents for the winter? I have for many years and feel it helps keep the floor warmer but now I have heard that it is better to keep them open to get rid of the moisture. I have black plastic laid over the dirt and have a sump pump that keeps the water from building up under the house.
KEEP THOSE VENTS OPEN!!!
Had your written from Minnesota, my answer might be a little different, but for folks who live west of the Cascades in places like Tigard or Seattle (wet but not too cold), the best advice is to keep as much air circulating through the crawl space all year round.
In Seattle we had a very cold snap (12F) a few years ago, and at that time I suggested the people pile some snow over the exterior of the vents. That way, once the cold snap was over, the snow would melt and the vents would be open. Had I suggested that people block the vents with newspaper (or whatever) some of the vents would still be blocked and the crawl spaces much damper.
In a crawl space with a sump pump the venting is even more important. If you need a sump pump, you have some moisture problems and you need as much venting as possible. By the way, are you sure that your gutter and drain system is working properly? How is the grading around the foundation? The following page on Basements might be useful.