Mobile:
Click More in bottom right corner
Go to Parts Inventory tab
Select Part to add Vendor(s)
Click 3 dots (⋮) next to Restock in top right corner
Select Edit
Select Vendor(s) to add in Vendors field
Click Done in top right corner
Click Update in top right corner
Web:
Go to Parts Inventory tab
Select Part to add Vendor(s)
Click 3 dots (⋮) next to Restock in top right corner
Select Edit
Select Vendor(s) to add in Vendors field
Click Update in bottom right corner