I have a 1956 Hose with a light above the sink and a vent fan on another switch. Do I have to have an receptacle in the bathroom, i.e. can a bathroom only have lights and vent and NO plug?

Your grandfathered in, so your fine.

Where I come from, if it wasn't there when the house was built, then its essentially "grandfathered in" and you don't need to put one in. Kind of like me not needing mirrors on my 1971 motor cycle :) But I digress, again where I come from, code stipulates one must be installed for new construction. And also, here, if I was to upgrade my house electrical, panel and wiring, then I'd have to follow the new code and install one. And there is probably stipulations as to location, and especially the use of a GFCI outlet.

EDIT: ah, you beat me to it KISS...

Actually, I changed out the old fluorescent light above the vanity to a new halogen one. The flouro one had a two-plug in it but we never used it because it looked scary. Does the grandfathering count if I changed out the switched to newer ones and the light fixture but haven't changed out the vanity, shower, vent fan, etc?

I don't believe so. As far as I know, it would have to be an overhaul. If you renovated the entire bathroom and ran new wiring, and fixtures etc, and had a permit you would more than likely have to put in proper outlets. Just changing switches and fixtures won't make you require an outlet. Changing switches and fixtures typically don't require a permit. But I'm speaking from code where I come from.