There are no 'vegetarian' snakes, vegetarian essentially meaning they do not consume animal protein.
There are snakes that have varied diets (garter, ribbon, Nerodia sp. water), there are primary insectivores (Storeria brown snakes, ringnecks), and there are specialists (egg-eaters, Sibon, queen/mud snakes, many western ground snakes, some Heterodon). The problem is that most of the specialists take foods that are inconvenient or impossible to provide (centipedes, scorpions, freshly molted crayfish, snails) and your insectivores and generalists NEED a varied diet to do well if you're not feeding rodents, as most commercially available food items are very poor, nutritionally.
Rodents should not post much of a problem, as it's very easy to get a snake that is feeding on frozen/thawed rodents, so there is no need to purchase and dispatch of live mice/rats yourself.
Ribbon snakes are cool, but make sure you do a very varied diet...fish, worms, tadpoles, insects, and I would still provide an occasional baby (pinky) mouse. Feeder fish in particular have a terrible nutrient profile.
They are also very active, and should be kept in a large tank. Don't skimp on size on these guys. They also prefer a large body of water. Perhaps a 40 gallon breeder with a cat litter pan for a water bowl.
My first snake was a Rough Green Snake. He was more hands-off, didn't care much for handling, but he was a really cool display animal and an easy keeper once I had his setup going. Rough Greens eat insects.