Perform the following in the Azure Console:

1. Navigate to Azure Policy
2. On Policy Overview blade, Click on Policy ASC Default (Subscription:Subscription_ID)
3. On ASC Default blade, Click on Edit Assignments
4. In section PARAMETERS , Set Monitor Vulnerability Assessment to AuditIfNotExists or any other available value than Disabled
5. Click Save

References:

1. https://docs.microsoft.com/en-us/azure/security-center/security-center-policies 
2. https://msdn.microsoft.com/en-us/library/mt704062.aspx
3. https://msdn.microsoft.com/en-us/library/mt704063.aspx
4. https://docs.microsoft.com/en-us/rest/api/resources/policyassignments/get
5. https://docs.microsoft.com/en-us/rest/api/resources/policyassignments/create
6. https://docs.microsoft.com/en-us/azure/security-center/security-center-vulnerability-assessment-recommendations