Chop it up.
Cut out the sleeves and the collar. Hack it to pieces. Vent your anger at the people who gained weight between costume fittings, and those sour individuals who don't like their costumes. Forget your mountain of waiting projects and scissor that thing apart.
Take your traced, borrowed or stolen but certainly not bought pattern of a nineteenth century man's waistcoat and place it where convenient. Make it fit. Save the front seams and buttonholes if you can.
At the end of cutting you should have the all the parts for a basic, not entirely correct waistcoat- an easy way to spice up any man's wardrobe. You may have to add some lining to the back piece, if you take out the usual polyester lining like I do. Take off the plastic buttons and buy some metal ones at Hancock's when they go 50% off.
This is how I sew. Shocked?