You can have a seamstress open it at the seam around the midsection and just bring the two “halves” closer together. They’ll need to adjust the zipper, but that’s all pretty straightforward. Leave a seam allowance in case you ever need more adjustments. Just watch the length on the legs — you don’t want to end up with high waters.
But yes to truly get a perfect fit in the vest and flight suit cost me another couple hundred dollars when all was said and done… With garments like these in a costume like this one it’s honestly to be expected unless you’re getting measured multiple times in person during construction.