Do you have to be able to see the VPN status within NPM, or are you just looking to alert if/when a VPN connection drops?
The easiest way to get alerts for VPN status (if that's all you're after) is to use the built-in alerts in the dashboard.
If that rubs you the wrong way (since there are no options for alert suppression, escalation, repetition, or really anything useful at all), you can configure the Meraki devices to forward Syslog to Solarwinds NPM. Within NPM, you can then set up Syslog Alerts that perform various actions based on the source/message/etc of the Syslogs that come in. VPN peer disconnects/connects both generate Syslogs, so this option may work purely for alerting needs.
Another option is to put together a small script that runs every so often and queries the
/organizations/$orgid/appliance/vpn/statuses endpoint, then fires an e-mail if/when it finds something down. This will take a bit of scripting/REST knowledge to effectively implement.
As for your original question...Natively, that's not something Solarwinds is going to be able to poll for. You would need to dig through the Meraki dashboard MIB to work out what OID(s) to poll to get VPN status info. More info here regarding SNMP polling the Dashboard:
SNMP Overview and Configuration - Cisco Meraki. Note - You have to toggle the one of the SNMP drop-downs on for it to expose the MIB download link. You don't actually have to save the change, though.