Something like this? This API will allow you to set the MX IP of that VLAN.
I typically use Python so here is a link to the Git for that. (if you search the page for "updatevlan" you'll find that API) - https://github.com/meraki/dashboard-api-python/blob/master/meraki.py
Then here is the postman API documentation for that as well (the screenshot is from here) - https://documenter.getpostman.com/view/897512/meraki-dashboard-api/2To9xm#eaad6d04-2a0b-0c4b-fc37-87...
This applies to "Vlans Disabled: Use a single LAN configuration"
API call to getvlans for a MX single lan configured with an IP shows ['VLANs are not enabled for this network'].
The assumption is if getvlans can't see a configuration, then addvlan will fail as there is no 'id' or 'name' to reference.
There doesn't appear to be an API call for a single lan configuration scenario.
Where you at Meraki?