Science fiction, comic book, and anime conventions often use "con" in their names or as puns. WindyCon, Conclave, MegaCon, Worldcon, CollossalCon, ... Referring to media conventions as "cons" is pretty common in fandom.

Veronica Mars doesn't fit strictly into those categories, but the fanbase seems to have substantial overlap with genre fandom. It's jargon, but not particularly uncommon in its area of relevance.