Vail is...well...a pup, or the childish behavior of Silva packed into one small bundle. He's not a persona Silva would want to use in combat being that Vail posses 0 combat ability. He serves more as an outlet such as Sava or Sal.  He's what Silva wishes still possessed, innocence. He comes out normally when Silva is feeling strong emotions of happiness and playfulness. Vail rarely makes a presence as Silva has aged further and further but he does make a good messenger when the time calls for it.