With Vetsulin, you will need to make sure you give your cat a good feed before the dose and wait 30 minutes before giving the insulin. It is a faster acting insulin than Prozinc and can hit hard. Make sure you test around the +4 mark to see how low it takes your cat.
Here is a link to the vetsulin http://www.felinediabetes.com/FDMB/threads/beginners-guide-to-caninsulin-vetsulin.186099/
Actually I'd test about +2 with Vetsulin as it can onset very quickly and knock BG down quite a bit early in the cycle. Some cats hit nadir by +3.5 to 4 so getting an earlier test will allow you to steer with food if needed.