And of course the vinegar wash won't last long if the cage is left outside. I have heard of people sending their galvanized cages to a shop that does a powder coating type of paint job. I don't know how well that works, zinc tends to be a poor paint primer. The best situation would be to take the raw iron wire in to be nickel plated, and forget about zinc and paint altogether. Generally speaking, the safest metals are what you find in your kitchen.
Iron is OK, it just makes a mess when it rusts.
And aluminum is OK, except with tomatos and acidic foods. You may also have copper cookware. But the copper metal is on the outside, for heat conduction and appearance, not on the inside. However, copper is relatively safe since most homes have copper pipe for drinking water. Something like a copper bell should be safe for a bird.