What is the query to get the information about tagged Virtual machines ?

There isn't a log out of the box that has information on tags for Virtual machines. I would suggest to develop something on your. Create a workflow that daily goes trough all your VMs in all Azure subscription and stores which VMs are tagged and which are not in your Log Analytics workspace. That workflow can be a runbook, logic app or azure functions. Whatever service is best for you to develop on. Once you have the log in Log Analytics you can use the query language to find the information you need.