I have a A record pointing to Azure website IP address.
Now I want to direct a subdomain to a Azure virtual machine.
Can I create a NS record xxx.example.com -> xxx.southeastasia.cloudapp.azure.com?
Do I need to create any CNAME records?
Do I have to do any domain verification at Azure?
If you are just looking to direct a particular sub domain to a VM then you don't need an NS record, an NS record is for where you want to delegate responsibility for a subdomain to that VM, so you want it to act as a DNS server for that subdomain. If you do want to do that you need to re-word your question.
If your just looking to direct traffic for that URL to the VM in question then you need to create a CNAME to point to the name of the machine or cloud service, or an A record to point to it's IP. If you are usign dynamic IP's then you are best using a CNAME to account for the IP changing.
Note that if you have CNAME pointing to a cloud service (if you are using V1 VMs) then that is pointing to a the cloud service, not the VM and could be used to access any end point in the cloud service.
