FWIW.... I hope you already have some experience building frames, as well as working with bamboo. If you do not construct the bike properly, you could easily wind up kissing asphalt whilst riding 15mph or faster, hardly a pleasant experience.

Along similar lines, I'd be concerned about a bamboo bike without a downtube. Aluminum tubing is quite strong for its weight, and can be shaped and butted for additional strength; the top tube of a Swift is fairly large and robust. I don't know if you can find / grow a bamboo pole that will match the same strength as the Swift's top tube.

Also, let's face it... If you need to reduce road vibrations, an unsuspended 20" wheeled bike simply is not the way to go, and even changing frame material is not going to make a huge difference. If anything, cushy tires like Big Apples, and possibly a carbon stem, carbon handlebar, gel padding and good bar tape might do the trick. But unless you have a critical need, I'd look into either a suspended 20" folder or, better yet, something with bigger wheels.