This one is pretty easy to pop and can be done after reaching Ant Hill.
*** Spoiler - click to reveal ***The two vendors will be the Shield representative in the Helicarrier's hangar Sidney Levine and Maddie Cho in Ant Hill.
As Mumblin 4man commented, when you beat the campaign and start the post-game activities, please note that SIDNEY the shield rep will permanently move together with Agent Morales to Substation Zero which you reach as part of the mission chain 'Back in Action' from Nick Fury. It's on the PNW region.
Note:I do not believe the leg-less robot Roy or the cosmetics vendors Chastity McBryde in the helicarrier count towards this achivement but I'm not 100% sure right now. If anybody has feedback on this please let me know
As soon as you buy anything from them both this should pop.