This requires your phone to be rooted.
Open a file manager
Navigate to /data/data/com/viber.voip/databases
Deleted the file /data/data/com.viber.voip/databases/viber_data
y of contacts stored in this db)
Once this is deleted, inside viber > contacts, all contacts are gone, now press menu and select update contacts.
All contacts are back and mapped correctly to conversations.