High, low or medium button on a waistcoat?
How you you determine which is wear on you?
Button position is generally fashion driven. So staying classic could be better in the long run but is up to you. I prefer the classic height or a slighty higher button position, or front opening (be careful at these are slighty different with the front opening obviously a 1/2" maybe higher then the top button position) because I am more traditional in dress and always prefer the styles and nuances towards the late 19th and early 20th century.
So where to judge on position of waistcoat button?
For me the general rule is to find the bottom of your sternum (this can be done by using your finger to feel where the bottom/end of your breastbone is) and use this as your lowest top button position, unless wanted to be trendy then could be further down if the trend is that way inclined. For medium top button position find the general medium spot (this is about 1 1/2" from the bottom of sternum if you are of average height 5' 8" to 6' so please expect more/less if taller/shorter than this). For a high top button position find a postion roughly 1" above that medium point.
For an easier although still quite general rule to determine top button position on your waistcoat refer to the picture below. Have drawn lines and suggested lines of sight to gage it against. Is still very general and can't be certain for every body type and height.
So here are the three looks that are not over trendy.