I don't have a vest but...
In most states (all) your dog does not have to be certified. Most places that you pay to have your dog 'certified' are quite simply ripping you off.
Yes, anyone can indeed put a vest on a dog and call it a service dog and depending on where you live people are not allowed to question this.
For example, where I live my dog will not need to have vest, patches, or anything else that identifies her as a service dog. If anyone asks I will simply say that "She is my service dog." End of story.
I do wish I had a spare vest to send, I will keep my eyes open for one, I think they help clear up a lot of confusion. You might consider this as a alternative to a vest.
I'M A SERVICE DOG - vest clip instead of patch optional ADA on back | eBay
You can also sew a patch onto your dogs collar. You can get vests or harnesses for under 30.00 on eBay but if that is too much money then that is too much. Just keep in mind that you while a vest is helpful there are alternatives that are less expensive.
More importantly, know your rights. I think that will help you quite a bit.