Hey burnt_casadilla,
Just a few things:
You shouldn't need alienCount or alienPos. Rather, you'll want to have a List<Alien> that stores its Position, so that it can Draw itself with only spriteBatch passed in as a parameter.
The reason for the error, is that you're actually calling Vector2.Add (which adds together 2 Vector2), since you're referring to the item in the List itself by specifying the index.

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