From a scientific standpoint, vegetarianism is not an evolved eating classification, it is a cultural, learned behavior. I believe you must be thinking of carnivores when you mention the length of digestive tract. Naturally, a rat's system would be a trifle shorter than a humans. They are a combination of the flat crushing molars of the herbivore, and the tearing cutting teeth of the carnivore. They would have had to have been given a choice at an early age, grouped with rats that ate vegetarian diets and rats that obtained their protein from mixed sources.
Rats learn food selection behavior in a group context.
Humans have a bit more cortex than rats and make the choice concerning what to eat based on information. I do believe that a a well-balanced vegetarian diet is a very healthy diet. I am just one contributor. Please doen't take my statements out of context.