It was this guy:
Charles II, huge royal babe and 17th-century fashion icon.
Yeah, Charles II basically invented the three piece suit. He was intensely fashion conscious because he didn’t want to be seen a Frenchified(not a scientific term) and yet France was consider ultra fashionable. So he had this problem of wanting to be fashionable and wanting to look good and avoid the puritanical looks of Cromwell, but he also knew that if he came back to England after exile in France looking like a French king, things would not go well for him. So he came up with the most fabulously English seeming outfit that quickly became ultra trendy and with in 20 years everyone was wearing the justacorp and vest combo Charles came up with, including French people. Fashion at its finest!
So really the vest came about because Englands greatest party king didn’t want to dress like everyone else was and needed to stick it to the super square Oliver Cromwell by looking FAAAAAABULOUS!
Charles, I thank you.