Take a look at Google Compute Engine. You'll pay by the drink, metered on your resource usage. Infrastructure cost drop drastically and you'll be on "GOOGLE's" gear / cloud. As an example, take a look at GetCloudCenter.com - the portal is running on GCE using LAMp servers, PHP, not sure about the DB side of things. They offer a demo portal to spin up and sell DaaS. Best of luck!
Throwing my hat in the ring and hopefully give you something of substance in the process.
Q: what provider are you running your VPS on and how?
Ex: WHM with cPanel and Wordpress 32GB RAM 4 CPU @ 2.8Ghz
A: we use GCE google Compute Engine, pay by the drink and zero slow down.
Q: Have you considered splitting things up? User Management server (AD ish), SSO to Wordpress & other DB servers, with hand offs and API calls to each other?
A: having each VM or VPS run side by side in a VPC will make for quick light work.
Q: you mentioned legacy Wordpress... Older version or??
A: We don't have any clients with WP issues on speed, it's usually the horse power of the VPS / VM and potentially plugins that are not quite up to par for a use case. Running WP on a pure LAMP makes for huge differences in performance.
Over all, I would recommend isolating servers and use them for specific tasks only and let the VMs do that one job very well. API to communicate between the VMs as needed.
Lastly, as for the ad engine - you may wan to look into an OpenX server.