The way that Microsoft does Azure pricing (or really any of their pricing) always has some odd caveat that makes you take a minute and think about it.
We ran into similar issues having a variety of subscriptions and needing a vMX100 at Azure. I ended up configuring the firewall more than 1 time but it did work in the pay as you go and I believe we are now under a CSP, or have a CSP, and still use the vMX100. It did not work initially. One of the two methods did not work and I had to contact and Microsoft for them to do something on the back end. After that it was smooth. The other times it went down were because of a mistake (one time) and the reliability of Azure.
You also have to pay attention to the area you are doing stuff in (for example: East != East2). One of the times our VM went down was, I assume, a configuration change. It was not by me but maybe someone on our Apps team or MS and it rendered the VM useless. Not really sure what happened. If I had to guess - storage disconnect via a network disconnect. Can't run without legs. A rebuild sync'd everything rather fast though. Rebuild is faster than restore.
The last time I went down was updates. I've noticed a lot of my issues were fixed when I ran beta on all other Meraki gear. I was not aware that the vMX100 doesn't like updates and Azure. I would imagine AWS is just fine. Therefore, another tip for you is do not update the VM as even though updates are pending they are not supported past 12.26 currently. This is, of course, just for Azure. This will eventually change I'm sure but as far as I know 12.26 is the max version you should use and be running on for the smoothest ride.
Edit:
Thinking about our configuration I also wanted to mention that you can't really set the WAN IP on it and it is whatever Azure gives you. I think it changes so if you add the site as a hub to your mesh Client VPN from another site or use the Dynamic DNS or something. However, I haven't tried that as our VPN runs from another branch.
I also used 1 /24 for private networks but all of the networking I can do from Azure I do from Azure and use minimal settings on the vMX100. Its nothing against Meraki - its really Azure.
I hope something helps you here. This can be done but no matter what man, it will feel a little hackish as Azure in general feels hackish (IMO). The vMX100 is solid - Azure is not.
Edit 2:
Also, make sure you are following this:
https://documentation.meraki.com/MX-Z/Installation_Guides/vMX100_Setup_Guide_for_Microsoft_Azure
Do the Azure part first. It should be a template. Note my tips from above. The subnet should be the one you want from Azure and it should also go under the Firewall part once the VM is up. Try one subnet and defaults before you go crazy. You also have to Mesh for the full effects.
The token is created in your Meraki Dashboard allowing the Azure Hypervisor to createa VM that shows a Virtual Appliance in Azure on your Meraki Dashboard. When you make certain changes you need API access from Azure to the Dashboard and that key is a secure way to do just that.