I didn't have the vector in my shop either after completing it but yet I did have the new flamethrower(not flamebearer, other one) but once I quit the game completely and started it back up it was in the shop. It's obviously a SMG so make sure you're looking in that section not the star!🟐 and restart your game!
Make sure you restart the game first then check before posting!
I'm assuming a lot of people are thinking it should just pop up in the store while your playing, since well the Flamethrower did but that's not the case for some reason most likely cause your in arcade? Cause for the Flamebearer cool little side notifications popped up when you completed a step or whatever? or maybe it was after I clicked complete/(aka)redeem which this challenge did not have that