Obligatory explanation of Miku, etc.: Vocaloid is a voice-synthesizer software. The Japanese releases of Vocaloid software are often associated with an anime-style character, and a fandom has sprung up around both the music made using the software and the characters. The most famous character/software package, Hatsune Miku, is getting a US release at some point this year, and they were hyping her like mad here. There were Miku-festooned cars in the convention center. There's a concert tonight, which I was only mildly interested in (and which, if past history with AX sound systems is any guide, would probably have killed my husband) but which was generating a lot of excitement among existing fans and some interest among others. Will got tickets, for instance. Here's an example of an earlier concert, if you're wondering how exactly this is done with a fictional character.
( Anime Expo happenings, including cosplayers, shopping, a bomb scare, my discovery of how very much I hate Hetalia, and more )
I know, what's wrong with me that I go to conventions just to shop. But the thing is, even setting aside Artists' Alley, you're never just shopping; if nothing else, you have to stop every three steps to stay out of, or detour around, the photo someone is taking of a cosplayer. You see the costumes, you hear other people reacting to what they see, you go from booth to booth growing progressively more smug as you realize you snagged a great bargain, you end up making conversation about your Murloco's shirt with strangers (okay, those last two were just me) - it's not the same experience as shopping for the same items at an individual store.
Edited, later: It seems likely that the swastika-wearing cosplayer might actually have been playing a particular Hellsing character, who otherwise matched the description. I stand by my dislike of Hetalia, though.