Modifying my grandfather’s late 1970 sportscoat

[MacMillan of Rathdown]

I will agree with the others who have answered that it does look a bit snug. As far as your other questions, the side vents are actually appropriate, and I would go with antler or bone buttons with the tweed. My tweed trouser sportcoat has leather covered buttons, but all the tweed kilt jackets I have seen have real or imitation antler or bone buttons.

Actually, leather buttons are perfectly acceptable for day wear and in mind they are preferable to plastic (read fake) "horn" or "antler" buttons.