Yep, simple and straight-forward: Which are the main veg*n organizations, collectives, etc. in Latin America? In which countries does each organization work?
Answer to the comments. Since Riker asks for the meaning of "main" word I suggest to mean it as in the common language; so this would mean "more popular, bigger, most known, most active organizations, etc." Additionally a look on the definition could be useful: "chief in size, extent, or importance; principal; leading."