In a medium bowl, place avocado halves and lime juice. Use a potato masher to lightly mash avocado into large chunks. Add onion, jalapeño, cilantro, salt, garlic, cumin, and vinegar (if using) to bowl.
Use the potato masher to stir and mash guacamole to desired consistency. I prefer my guacamole on the chunkier side, but some people prefer smoother, so keep on