@mlj 

>Can I set up the same SSID on these that the VR2800 is broadcasting?

Yes, you can. It is like taking two different standalone routers and configuring them with the same SSID - nothing stops you from doing that.

> If so what happens if there is an "overlap" in an area where both Deco and VR2800 wifi are visible ? 

Mobile device will treat Deco mesh and VR2800 as two different routers, both broadcasting same SSID. Device may roam from one router to another, but during that it'll briefly lose WiFi and Internet connectivity. If this is OK with you, that's the only issue mobile devices should experience in this configuration.

For seamless roaming inside WiFi mesh, you will need to disable WiFi on VR2800 and attach M5 to it with short cable, to provide WiFi coverage in that area.

  @mlj Many thanks !!