Hi Edmond,
You can try to go to the vendor record and then change the party association to be the same as the new created legal entity as work around.
I had also written this in my book related to merging GAB entries. See below:
Internal party organizations
It is not possible to do a merge with internal party organizations. Legal entities, teams or operation units cannot be merged. One of the reasons is that you can have defined organization hierarchies. If you have an organization hierarchy with e.g. business units, the records to be merges could be both part of a current, past or future hierarchy. Merging would cause a duplicate reference in the hierarchy which is not allowed in standard Microsoft Dynamics AX.
Suppose you have a legal entity Contoso Retail USA and also a party with the same name used as customer, you will receive the next error while the merge is being validated. “Internal organization party records cannot be merged.”
Workaround
To work around this validation, you can perform the next steps. In this example we will solve it for the customer Contoso Retail USA.
1. Navigate to Accounts receivable > Common > Customers > All customers.
2. Find the customer with the name Contoso Retail USA.
3. Click Edit or double click the row with this name in the list page to open the Customer details.
4. Click the button Change party association which can be found on the General tab page.
5. Find and select the New party record. In this example it will be Contoso Retail USA.
6. Click the Select button.
7. When you are back on the Change party association form, review the party name and number.
8. Click OK to change the party association.
The customer will now be linked to another party. Note that addresses are linked to a party. If the addresses were different, the customer will have a new address. You can create multiple addresses on a party. Per legal entity you can set a different default address.
Using this procedure will not merge all related records in the various tables like the merge feature does. Check if there are e.g. addresses or activities related to the party which needs to be moved or deleted before executing this procedure.