Hi,
In Dynamics 365 Finance and Operations (D365 F&O), there is NO direct functionality to "merge" two vendors in the same legal entity.
If you are were trying to merge two vendors across different legal entities, it could be made possible by assigning both of them to the same Party resulting in a merger of both of them.
But as you told, you have two vendors added in the same legal entity then there is a workaround to resolve your issue as below:
1. If the second vendor account has no transactions yet, delete that vendor.
2. If both the vendors have transactions and data, then you can consolidate or reassign the vendor data into one of them by performing several steps manually. This typically involves ensuring that all relevant transactions, vendor balances, and data from one vendor are moved over to the other vendor, and then deactivate/delete the 2nd vendor.
Hope this satisfies your question in detail. Feel free to ask if you have any further questions.
Regards