I will go with CNAME because this is what has been widely used by Windows Azure users around the world, however, as a guarantee of Windows Azure that the VIP will not change during the life of the service until you delete the service, so you can use it. I saw how users started using it as well.
-> About your claim that the VIP has changed, even if you did not delete your service, I would say that this should not even be with a trial account.
- How did you deploy your application? If you deploy the application in such a way that the previous deployment is removed and the new deployment, this can lead to a change in VIP. For example, deploying from VS, delete the existing deployment and then create a new one which will lead to the same behavior. It is always good to upgrade an existing deployment and make sure that the process you are using is updating (without deleting + a new deployment).
If your case was not above, and you are sure that your VIP has been changed without deleting the existing deployment, you should notify Windows Azure Support, as Windows Azure declares that the VIP will not change during the service life if it is not deleted.
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