When gentlemen wore pocket watches, they invariably wore waistcoats. Otherwise where would they put their watch? When wristwatches caught on, that was the beginning of the end for waistcoats. I think it's quite acceptable to wear a waistcoat without a jacket, although at one time it may not have been.

Perhaps waistcoats might come back as a fashionable way to wear a mobile phone?

A word of caution to our American friends. Yes, we know you call your waistcoat a vest, but did you know that in the British Isles a vest is what you might call an undershirt? If you said that you wore your vest outside your shirt people might think that you dress like some sort of comic book superhero, or that you are simply a bit eccentric.