Again, there is no such thing as vegetarian poke.
That aside, in order to serve the octopus raw, you need to tenderize it by beating it with a peeled daikon radish. It imparts flavor while tenderizing.
Then slice the octopus extremely thin and allow the citrus to "cook" it like ceviche.
Mix with tomato, red onion, green onion, soy sauce, sesame oil, and chili pepper.
Just before serving, sprinkle with a nice course salt.