The demise of the waistcoat (along with two pair of trousers) was due to the necessity of rationing cloth during WWII. Men became used to buying suits w/o waistcoats and the extra trousers, and both retailers and manufacturers discovered that they could charge the same when supplying just a jacket and trousers. It was the lack of waistcoats that caused menswear to become less "formal", not the other way round.