Essentially, you have to re-direct your viewpoint so that your diet does not require "replacing" animal protein, but rather is balanced of itself. We need about 5-10% of our calories as protein for basic sustenance. (Athletes and other highly active people might need a slightly higher proportion to allow for cell regeneration.) If you base your diet on staple grains (corn, wheat, oats, etc.) plus legumes (lentils, beans, chickpeas, soybeans, etc.) you will find that it's super easy to achieve that balance. For example, plain ol' rye bread has 13% of its calories as protein. Broccoli has over 30% of its calories as protein!

So basically, if the diet is based on a wide variety of whole foods, you don't have to worry about protein!