Looking at this from a mechanical point of view. If you wore a waistcoat on almost a daily basis and wore the "t" bar on the inside wouldn't this create a potential wear point. Every time you move the waistcoat will rub against the shirt. If the "t" bar is between the waistcoat and shirt it would move with the waistcoat causing friction on the shirt. Even if the "t" bar is caught between the button hole flap and the button flap there would be friction. The friction would wear holes.

This might not be a significant issue, but, depending on the design of the "t" bar it could be a problem.