Ok, currently the scope of that page is the list of Client VPN users vs. 'known' users that may have got into the list a variety of ways. I think It's really there to allow granting/removing Client VPN access, hence the lack of any rich API around it. If you do not have any guest users (i.e. all will have an admin account), you can use https://developer.cisco.com/meraki/api-v1/get-organization-admins/ which gives you all admin users. If there are guests, I'm not sure, if doing the https://developer.cisco.com/meraki/api-v1/get-network-meraki-auth-users/ call, which you (edit) might need to on all networks to build up the full list, isn't working then I don't think there's a way in the API. In that case one option is to 'cheat' by making a screenscraper, or to poke around in the page code and find out the URL used by the "Download as CSV" button, then use that to grab the list as a CSV you can process.
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