Young Man with an Open Vest
This young man in a frock coat has his vest buttons undone in a style typical of the 1860s.
The tendency to leave vest buttons unbuttoned was widespread in the 1860s, along with baggy clothes, unkempt hair and sloppy ties. It was all part of a notion that if a man spent too much time on his appearance, it indicated a shallowness of character.
That changed in the 1870s, when it once again became the fashion for men to take some pride in their appearance.
Note also the tape edging on his coat -- this was probably silk. Such edging is a very common feature of 19th and early 20th Century coats and vests.