Once your account is deactivated, there is no way to restore any of your user data. Viber does not store any of your message history and is unable to retrieve lost data. Re-registering to Viber with the same mobile phone number will not restore the previous account history.
Create a backup of your Viber chats for safekeeping prior to deactivating your account.
Source: Handbook on Information Security, Sri Lanka CERT – CC