I think it is bugged. I found a reddit thread yesterday where a handful of people were trying all different weekly challenges (gambit, vanguard, etc) and none were counting towards the Guardian Rank 6 challenge progression except for the Banshee weaponsmith bounties challenge on the tower. You can complete 8 bounties from him and get a weekly challenge which counts towards the Guardian Rank Vendor Challenge for rank 6. But, it is a weekly challenge so I'm currently having to wait until next week to do the same Banshee bounties again for a second weekly challenge.