A Vanilla Townie is a town role which lacks any additional powers or abilities aside from voting and posting in-thread. They are the most common role in Mafia, and are also the most important Townie role, because Vanilla Townies have only their wits, reason, and ability to find scum, without relying on night actions or other role-based information. Thus to play well as a Vanilla Townie is to be considered a "good" player of Mafia, since it is the truest test of ones skills.
Likewise for balance issues, too many Power Roles can create a situation of chaos, which oftentimes overly benefits the Town, unbalancing the game and causing the scum to lose. So it is recommended that most mods make their games with at least as many Vanilla Townies as there are mafia, in order to allow the mafia to not only avoid a no-win massclaim scenario, but to fakeclaim Vanilla without sticking out.
Playing a Vanilla Townie can sometimes be "boring," as you lack any cool role or claim. However truly playing Mafia can be a very fun and rewarding experience when given the proper effort.
The opposite of a Vanilla Townie is a Power Role. People often use the term "Townie" to refer specifically to Vanilla players. Oftentimes people call Goons "Vanilla Mafia," since they lack any special Mafia abilities.