From the moment you have sent the message, it will be delivered to the receiver. If the receiver didn't saw the message, then it will be marked as Sent. When he'll see it, the message changes its status to Delivered. According to the FAQs on the Viber website, the message can be marked as Delivered without being seen. This is the way the system works. However, if you send the message, and then disable your account, the receiver will have the message to its inbox.