I would imagine it depends on how long you want your vest. If it will be made to partially cover your hips, then you might need to add some shaping to the bottom. However, if it's somewhat shorter, then you can probably get away without sizing that part of the garment. I would measure around your waist/hips where you expect the vest to end. Then compare that measurement to the size of the garment (or a similarly fitting garment--doesn't have to be this one) a few inches below the bust. If these two measurements are similar, you probably don't need sizing for your hips. If they're different, you probably do need some increases.
Of course, you can add a border to stockinette stitch to keep it from curling, and if you go the stockinette route, you may need to add some kind of border around the underarms and neck, too. Those will curl in straight stockinette stitch, so beware!
If you use entrelac, I don't see any reason why you can't include slits--anywhere and as many as you like! :teehee: They could certainly help with the fitting issue.
Do you have a similar vest that fits you well? Or any other type of sweater or shirt? I'm thinking you might use such a garment for a guide.