eAthena is made to work with the latest kRO client and will not work with other language clients. If you want vietnamese in the client, you'd have to make a vietnamese language pack for your server.
What you would need:
1. A custom data folder with the vietnamese item names and such.
2. The vietnamese skin for RO.
There are packet differences in the clients (as you could see from the error), and since eAthena is coded after kRO packets, that is what it will work with. You will get the same answer on the eAthena boards.