@Hajny 

While VR400 supports OneMesh, the Deco mesh is different technology and they are not interoperable - see the difference and OneMesh FAQ - Q3/A3.

The simplest thing you could do is to name the VR400s SSIDs the same as Deco E4 SSIDs.

But this way you'll still have two separate networks - one from the VR400 and another one from the Deco E4 mesh, introducing a double NAT as well.

What I would do is to set VR400 in bridge mode (with wireless radio disabled) (if that's possible with your ISP - example) and retain your Deco E4 original configuration.

If that's not possible you could use VR400 in DSL modem router mode (with wireless radio disabled) and set the Deco in AP mode.

In both cases you should have a mesh network with Deco E4 the same way as before without introducing a double NAT in your network.