Unbuttoned Casual Shirting

Hi all, a problem I have been having - I like to wear fitted casual shirts unbuttoned for that slightly 'undone' look however when I wear a fitted plain white t underneath, I find this really tight on the armpits .
I guess the closeness of both fabrics to the pit. I am thinking that a vest might be the better option however I am more inclined towards the traditional sleeveless vests compared to the wife beater type. What are your thoughts/ solutions

Personally, unless you have the body of a Calvin Klein underwear model, I wouldn't wear anything too thin as an undershirt if you're unbuttoning the outer shirt. Most of the sleeveless/tank top undershirts I've seen seem to show off too much underneath!