Your understanding is correct. If both MX WAN ports are on the same L2 from the ISP and the /29 is delivered on the wire, assigning VIP (.4) plus additional public IPs (.5, .6) works: VIP handles AutoVPN and Client VPN without interruption, while DNAT rules against .5/.6 float seamlessly during warm-spare failover. Just add the additional IPs under Security & SD-WAN > Configure > Firewall, set up your DNAT rules, and point all VPN traffic at the VIP. Refer to the Meraki Documentation: https://documentation.meraki.com/MX/NAT_and_Port_Forwarding/Port_Forwarding_and_NAT_Rules_on_the_MX https://meraki.cisco.com/blog/2014/08/1many-nat-for-meraki-mx/
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