Place octopus in a pot and add enough water to cover; add red wine vinegar. Bring liquid to a boil, cover pot, reduce heat to medium, and simmer until a toothpick goes through the thickest part of the octopus smoothly, about 30 minutes. Remove octopus from pot and chop into bite-size pieces. Discard water-vinegar mixture.
Combine orzo and chicken broth in a small pot over medium heat; allow orzo to absorb the chicken broth, adding more water, about 1 tablespoon at a time, if orzo gets too dry. Cook until orzo is tender yet firm to the bite, 5 to 8 minutes; drain.
Heat oil in a skillet over medium-high heat; saute yellow bell pepper, mushrooms, and olives until tender, 5 to 10 minutes. Season bell pepper mixture with black pepper.
Mix orzo, spinach, and octopus into bell pepper mixture until spinach begins to wilt from the heat of the orzo. Add lemon juice to orzo mixture; top with feta cheese.