That's a fairly "relaxed" definition of semi-formal and it sounds as if you'd be fine in a shirt and tie, with the vest being a nice touch, too. If you already own a tux, that contemporary jacket might fit a nice niche for events where the tux would be too much. (And one day, you might want to look into a Black Argyle jacket or PC coatee so you can kilt up for those formal events, too )