Washing a vintage bike—special considerations?
Cleaning a vintage bike isn't that much different than cleaning a new bike, with perhaps two caveats:


Vintage bikes are usually made of steel and the frame drain holes may be plugged or non-existent. I wouldn't use water directly (from a hose) directly on a vintage bike but instead only use wet rags and wipes. Don't spray (citrus-based) cleaner directly on the bike, spray it on the rag and then wipe the bike. While cleaning the bike, I'd see if there aren't drain holes at the bottom of the bottom bracket, chain stays, and front forks and make sure they're clear. 
Vintage bikes usually have decals that might be easily accidentally removed or damaged. I'd be particularly careful cleaning around these decals. If you like how they look and don't care about restoration value, I'd protect the decals by spraying them with a decal-safe clearcoat (or putting an overlay of helicopter tape) - but be aware that this will destroy their value for a conservationist, so do it if the bike is emotionally valuable to you, but not particularly rare (such as say, a 1984 Trek).