Though it's totally dairy-free, traditional mayonnaise is not vegan. Regular mayonnaise is basically made by emulsifying a raw egg with oil, and eggs are decidedly not vegan. So how do you make mayonnaise if you're not using an egg, and what's in vegan mayonnaise, anyway? It turns out that the list of ingredients in vegan mayonnaise is very similar to that on a jar of traditional mayo, and it includes oil, vinegar, and lemon juice. The big difference between vegan mayonnaise and traditional mayo really comes down to the egg substitute. "We use vegetable proteins in place of eggs," explains Bob Goldberg, cofounder of Follow Your Heart, the manufacturer of one of the OG vegan mayonnaises Vegenaise.
That plant protein is what gives vegan mayo its texture and consistency. "The protein helps the oil and water to mix," Goldberg writes in an email, adding, "and by using vegetable proteins such as soy or pea, we achieve the same smooth, creamy texture without the added cholesterol or adverse impact of animal agriculture." And other companies besides Follow Your Heart use plant proteins as a substitute for egg in their vegan mayos. For instance, Hampton Creek’s Just Mayo uses a pea protein rather than egg in their vegan mayonnaise, and Earth Balance relies on soy protein to add heft to its organic vegan mayo.
But just because your jar of vegan mayonnaise doesn't have eggs in it, doesn't mean you shouldn't treat it as a perishable food. In fact, "Vegenaise is kept refrigerated from the moment it’s made to the moment when the jar is empty," explains Goldberg, because the company doesn't use any preservatives. Other vegan mayonnaise brands, like Just from Hampton Creek, are sold at room temperature but should be refrigerated after it's been opened, just as you would store a jar of traditional mayo.
At the end of the day though, vegan mayonnaise is just egg-free mayonnaise so you should use it just as you would any other mayonnaise—and there's no reason to be more afraid of it, either.