Viber is an amazing app, no matter if you delete the app from your mobile it keeps your backup and whenever you install in other phone and provide the previous number it will restore your information.
Viber Also allows you to Backup your Data. In this way all of your data will be saved in your Email I.D or any storage Option you choose.
In case if you want to permanently deactivate your viber account then use the following steps.
Launch Viber on your android phone
Click on "Menu" Option present on bottom right corner on main viber page
Open the settings
Click on "Account" option
Now click on the option "Deactivate Your Account"
Now Choose your Country first where your account is based upon
Now Write your phone number to delete your viber account
Now click on deactivate your viber account.
In This way way your account will be deleted permanently.