This should be easy to do with the HTML ABBR tag, like this: <abbr title="full name">abbreviation</abbr>

I don't know if vBulletin supports automatically replacing certain pieces of text with tags (so that SFO would become <abbr title="San Francisco International">SFO</abbr>). It seems a bit far-fetched, but supposing it does, I would be in favor of using such feature this way.