I have some buddies who use their Mtn bikes to go "bikepacking." They load up their backpacking pack, strap on skis or climbing gear, and use their bikes to ride 20-30 miles in on fire roads and logging roads to access remote areas for climbing.

In the PNW this makes a lot of sense--several passes are closed during the winter, but the roads often become snowfree before they're opened again in the summer. Throwing everything on your bike to get to the trailhead you'd normally drive to provides access to areas too remote to walk in to, or too crowded in the summer. It's also the only way to access some of the best spring backcountry skiing in the North Cascades.

If this is the kind of thing you're thinking about, I'd recommend going the mtn. bike route instead of the vintage route. Cyclocross would be fine, but most cyclocross bikes only use a double up front instead of a triple, and you'll appreciate the lower gearing when you're slogging along with 60 pounds of camping and climbing gear. Something like the REI Safari or Koga Miyata seem like they'd be perfect for what you're describing.