None of the merchants added by Dragonborn have the quest perk investor. They also don't have globals for tracking investing in them nor correct leveled lists set up for their vendor chests. I'm thinking this is intentional, being able to invest in these merchants was never intended.
List of merchants' chests with LVLIs that should be removed:
DLC2RRMiloreVendorChest PerkInvestorRiftenApothecaryList
DLC2RRGloverVendorChest PerkInvestorRiftenBlacksmithList
DLC2RRFethisVendorChest PerkInvestorRiftenPawnList