I realize that it's not Japanese, but there are only three big reasons someone would use a V-sign.
1. As a symbol of victory. This was most widely used in the Western world during the second world war, and has become rather archaic now.
2. As a symbol of peace. Either because victory usually leads to times of peace, or because peace activists used it in the 1960s. This too has fallen out of wide use in the western world.
3. As an insult similar to the middle finger. This was never as widely known, and anyways you'd have to display it with your palm facing inward.
Since they clearly haven't won a world war anytime recently, and their palms are facing out, I'd say it's safe to assume they are using it as a peace sign. As far as I can tell they are not protesting any wars (though that might be a better use of their time.) I have to assume they picked it up from watching some sort of Asian program because Asia is the only place in the world where the V-sign (meaning peace) is still popular. Many people (especially the youth) display the sign in an effort to look more westernized. Popular entertainment picked up on this and incorporated it into many programs (including animation) and this wasn't changed when they were shipped to western viewers. Because of the popularity of this sign in many animes, and its infrequent use in the western world at the time, many fans of anime use it because it seems like a Japanese thing to do. I think the fact that they are also at an anime convention helps this theory. How many people do you see using the v-sign on a constant basis that aren't dedicated fans of anime? How do they stack up in comparison to the number of people you've seen who do use it and are fans of anime?
Edit-I also want to add that I have nothing against Japanese culture or anime, but people that latch on to things just because they seem foreign and exciting annoys me. Sorry if this turned into a huge argument.
That guy does have a nasty beard, BTW.
Edit 2-