Vibers Privacy policy is here.
From this I get that -
Among other things they will leak which of your contacts use Viber, and leak that you use Viber to them.
Disclose your Viber number to whoever wants it "they do not consider the fact that your Viber number exists on viber as private information"
Viber will share your name and photo with other parties, as well as your online status, the last time you connected and and if you have seen messages sent to you.
If you use billable servers, Viber will pass details on to those parties so they can bill you and provide other requested services.
They will allow other companies to track you on there behalf - ie use other companies for analytics - including Google Analytics which provides some detail about your usage habits to Google.
They will give up your information if required to by law.
Viber does use encryption, although the encryption method is not easily come by. They also have had issues with being hacked and leaving some data unencrypted.