I receive a few Vanguard Mentor Missives per week, and the exotic bounty for the Pocket Infinity has been available only once to me so far. As for the weapon, it's sort of nice, but it seems to have more recoil than any gun in the game. Also, this is a very expensive bounty. Depending on your circumstances, be prepared to spend 10,000 - 20,000 glimmer for the fusion rifles, special ammo, and weapon upgrades.