The first thing to do it put on a proper shirt and tie, dress shoes and a belt i.e. wear it the way you're going to wear it. You'll get a much better idea of how it's really looking. Then review what E.TF said!
If there is plenty of excess fabric in the side seams and the vent itself, your tailor might be able to let it out a bit, but it's tricky to keep it hanging right. But I suspect it;s just the wrong jacket. I know the centre vent is very common in the US, but it might not be right for your body shape, at least not in standard fits.