The user in question also needs to be set as a Rep or Master Rep for the vendor.
Navigate in the Admin CP to
DTO Vendor Tools=>Manage Vendors
In order to access the VendorCP a Vendor Rep needs to be assigned. Other Reps can be added by the Master Rep after the vendor has been provisioned.
Click on the [Vendor Reps] link for the desired vendor.
Enter the username of the Vendor Rep. The individual must be a member of a usergroup selected in the Usergroups having Vendor CP Access setting found in Manage Settings.
1. Set as Active.
2. If the individual is to be the Master Rep select Is Master Rep.
FYI, this is under the Manage Vendors topic in our extensive documentation.