Onions are the underground bulb of the plant. It is not a vine.
I don't know how far apart you planted the onion seeds or sets, but they need to be thinned as the bed of onions grow. Thinnning is the process of removing plants to allow more room for the bulb to grow. How large do you want your onions to be and you know how much to eventually thin the crop. You thin garlic and radishes the same way. When the onions get to the point of nearly touching eachother they are ready to harvest.