Last rode the BV trail from Vernonia to Banks a couple of months ago. The trail was in fine shape and completely rideable from start to finish on a cyclocross bike with 32c tires. A great trip!
I would not advise doing the full trail on a tandem. There is a section of deep gravel doubletrack that is a couple of miles long, along with with some steep, mildly technical rocky descents near the Tophill trailhead off of Highway 47. I walked my bike down this section, since I didn't feel like risking a fall so far from home.
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