formal evening dress: vest or no?

[sfb]

I know MacMillan of Rathdown said that a Gentleman should keep his jacket on at all times except when playing pool/Snooker or offering his coat to a lady (Sorry I can’t find the quote right now).Jordan

I think this is the one you're after:

"a gentleman never removes his jacket in public, especially when at table. He may, however, remove his jacket when playing billiards, or to cover a lady's shoulders should the evening air turn chilly."

I'm not sure that gentlemen are ever permitted to play pool, jacket or no jacket.