Rav Aharon Butbol was asked by a yeshiva bochur about the need to don a black hat and jacket for the purpose of bentching.. how necessary is it really?

Rav Butbol responded paskening that technically there is no need to wear a hat and jacket for davening. The problem is, which the hat and jacket resolves, that a head covering has to cover the majority of the head for it to be considered a valid head covering The modern bochurim wear such small kipas it has become necessary to wear a hat for davening to fulfill the halacha of having the head covered. Yeshiva bochurim, a ben torah, should wear larger kippas, specifically a size 6 or 7.

Rav Butbol said that for bentching there is a halachic preference to be "wrapped" - Ituf as we do for davening with a tallis. The jacket accomplishes the need for ituf, but it is not absolutely necessary rather only a midat chassidut.

Also, if someone accepted on himself to don his hat and jacket for bentching, after 3 times it does take the status of a neder and if one wanted to stop doing that he would need to do hatarat nedarim, unless he includes that in the hatarat nedarim done on erev Rosh Hashana. 

source: Kol B'Rama