formal evening dress: vest or no?

[JSFMACLJR]

From my childhood I have always been tought that a 3 button waistcoat is more formal than a 5 button waistcoat. So to me it is just a matter of how formal the party I am going to is.

And what about 4 button waistcoats?

[CMcG]

And what about 4 button waistcoats?

That is apparently some murky territory:

[creagdhubh]

And what about 4 button waistcoats?

Lovely tartan waistcoat, I too own a few-yet they are the three button version. The four is quite elegant. Well done!