Note: For step 2 & 3, you will need to use the old admin center page, which can be changed by clicking on the button 'Go to the old admin center' in the top right
In order to Migrate to VIA from Skype for Business Online fully, you need to perform 2 actions:
1) Remove the license from each "Active User" from the Office 365 portal (https://portal.office.com):
Select a user and then click "edit" next to "Assigned Licenses" on the right of the screen.
Untick "Skype for Business" from the licenses:
You must perform this step for EVERY user enabled in Office 365.
2) Remove the "Domain Purpose" for the domain you are adding to the VIA portal.
Click "Domains" in the Office 365 portal. Then select "Domain Settings" with the correct domain selected and then "Change Domain Purpose":
Untick "Skype for Business".
3) Navigate to the Skype for Business Online Contol Panel. Select "Organisation", then "External Communications". Ensure "Let people to use Skype for Business to communicate with Skype users outside your organization" is un-ticked.