I'm going to be running a game using the Dresden Files RPG and have a player planning to play a Red Court Infected. For purposes of creating and maintaining thresholds and magic circles are Red Court vampires and/or Red Court infected considered mortal by Dresden Files RPG rules? In other words, can they create a threshold for a house by living in it, can they close a circle against magical forces? If I'm not just missing something obvious and it's not in any of the RPG books, can you point to an example in the Dresden Files fiction that clearly establishes it one way or the other?
Red Court Infected are mortal, as are White Court Virgins and Changelings. The only distinctions Dresden ever really makes is whether or not people have powers and whether or not they have Free Will. Humans with powers (especially Wizards) are explicitly called out as mortal (hence the Venatori don't just let the White Council in on the Oblivion War). So, the powers themselves don't make one a non-mortal. And all the part-humans we've seen have Free Will, with their transformation to inhuman being a result of Choice. So we can reasonably infer that they also count as mortals.
Creating a circle of power does not require that one be a mortal at all, though. Certain Faeries use one in the Cold Days novel, for example. As does the nagloshii in Turn Coat. In fact, there appears to only be one canon example of a Pure Mortal doing so and whether or not he actually is "Pure Mortal" is probably open for debate. Honestly, I'd probably just consider them a natural part of "magical physics", just like ley lines and running water.
Thresholds, however, have no canon answer. The fact that Red Court Infected are mortal should theoretically allow them to create a threshold, however, we never actually see Dresden put in a situation where it comes up. Most of the time he's entering inhuman lairs, in which case he has other reasons to receive an invitation; or he's entering the homes of (presumed) friends. So we really don't know if part-humans produce thresholds. But since we know that powered humans (talents, Wizards) do, it would be reasonable to infer that part-humans do also.