Can you substitute red onions for vidalia onions?

Yes, the substitution can be made. However, it is important to be aware that vidalia onions are generally a sweeter and more mild onion and so you should start out with less of a red onion than you would with the vidalia to make sure that the end result is not too strong. Depending on the recipe, you can try carmelizing the red onion which will decrease it's "strength" to counteract the difference in flavors.