1. Take a backup of the Vendor Management database.

2. Run the following script on the database after replacing the UserID listed in both places with the UserID of the one who should be the administrator:

NOTE: Text in red may need to be changed to match your portal database naming convention. 

INSERT INTO [Production_Portal_VendorManagement].[dbo].[CategoryOwners] 
SELECT [ID], UserID, Firstname + ' ' + LastName as Name FROM Production_Portal_VendorManagement.dbo.[VendorCategory] CROSS JOIN [Production_Portal_PortalFx].[dbo].[PWUser] 
where UserID = 'DC65D14A-7C42-4008-A80F-A473773F507B' and ID not in (Select ID from [Production_Portal_VendorManagement].[dbo].[CategoryOwners] where userid = 'DC65D14A-7C42-4008-A80F-A473773F507B')

You can find the user id by opening the Portal Framework database, navigating to the PWUser table, and querying for the user in question by login, firstname, or lastname. 

Ex.) 

Select * 

From Production_Portal_Portalfx.dbo.PWUser

where login='portaladmin'