Their meatless menu, as it turns out, is really just a menu of bread, cheese, and sauce with a few vegetables here and there. It's not all that surprising since their brand is devoted to "meatcraft."
Their version of the roast beef sandwich? "A toasted sesame seed bun minus Arby’s famous thinly sliced roast beef, marinated and roasted in Arby’s restaurants every single day. Available in Classic, Mid or Max sizes." So essentially just a piece of bread.
Not very inventive and perhaps really just an early April Fool's Day joke. Apparently, vegetarians only eat once every four years. But, if their vegetarian menu goes well today, their chief marketing officer said they will bring it back on February 29 every year.