No your service dog does not have to be marked or wear a vest in order to go int a store in Minnesota. Federal law prohibits store owners from asking someone to leave if they bring in a service dog. Now here's the catch though, say your dog is not marked, you can't prove that he/she is a service dog, the store owner may ask you to leave. The majority of service dog owners or service dogs in training choose to wear the designated vest to avoid any conflict. In addition service dogs behave differently than a normal pet dog. A savvy store owner can sniff out a dog that is not a service dog. I would recommend that your service dog wear a vest to avoid any conflict that may arise.