This is the theory that I've been thinking about the most, and it absolutely fascinates me. 

From 20:05-20:25 in the video, Deceit and Virgil have an exchange that really made me think.

Here's the exchange:

D: "You are not a beacon of truth."
V: "Yeah, because I'm wrong a lot."
D: "Oh, so you've never been reluctant to share anything with the group, then?"
V: "Don't."
D: "What, I just meant your name."
V: "Don't!"
D: "Maybe that's why it's so easy for you to recognize me as I am. Like I said before, it takes a liar to know a liar."

The first time I watched it, it didn't stick out to me too much, but then I saw someone on Tumblr point it out, and I couldn't help but go back and rewatch it. That was when it stuck out, and the gears started turning.

So, my theory with this is that Deceit is trying to hint that Virgil knows more than he's letting on. Now, what exactly Virgil might know, I'm not exactly sure, but I have a few ideas.

The first is that Virgil knows more about Deceit than he is letting on. It makes me think that the two of them have a history of sorts, although the history wasn't pleasant, which is made evident by Virgil's behavior towards Deceit throughout the video.

One thing that really stuck out to me that might support this is when Deceit said to Virgil, "Glass houses, Virgil." That stuck out to me the very second I heard it, although I have no idea as to what it might mean.

The second is that Virgil knows more about the Dark Sides, and he is reluctant to share. There have been a few people on Tumblr that have theorized that Virgil was a Dark Side before he showed up, which means that he knows the Dark Sides. Someone else on Tumblr mentioned that Virgil's name is different which may be from him being a Dark Side before joining. 

One thing that supports the whole theory that Virgil knows more about the Dark Sides is when, nearing the end of the video, Virgil said that Thomas should never let "him [Deceit] or his friends" stay as long as Thomas had let Deceit stay. 

The last one is that Virgil's purpose may be more than just representing Thomas' anxiety, but he isn't sharing that. I didn't think about this after watching the video until I saw someone mention it on Tumblr, and that got me thinking. What would Virgil's other purposes be that he doesn't want to admit? Like, how Logan is in charge of so many things, which he lists when Deceit is questioning him, is Virgil in charge of other things?

I haven't been able to think about that too much since realizing that, but that would certainly be interesting to explore, and it'd probably include me rewatching a heck of a lot of the episodes to see how Virgil acts.