There's an identical thread on Fatcogs. I think it comes down to the question of, what do you want to get out of the bike? I've seen a lot of chatter on this site (and others) about people wanting that "look and feel" of an old bike but without the penalty of a rigid fork or the poor performance of cantilever brakes. Etc, etc.
Ultimately, you can't turn a donkey into a racehorse no matter how much hay you feed it. By putting a suspension fork on an old Yo, you're sacrificing everything that's great about the bike, like the aesthetics and ride quality of the Yo fork which compliments the great geometry of the frame. This is a worse travesty with a vintage suspension fork because the loss of the steering capabilities of the fork will far outweigh any gains made by the shitty suspension.
Own a Yo (or any old bike for that matter) and ride it rigid and accept and appreciate the ride quality for what it is. Own a suspended, modern bike for comfort.