@Private3 

TP-Link VR600 router and Deco mesh system each run their own independent WiFi network. Deco mesh does not extend VR600 WiFi network and there will be no seamless transfer of device between VR600 WiFi and Deco mesh WiFi.

It is recommended to turn off router WiFi, in your case that would mean turning off WiFi on VR600 router and letting only Deco mesh provide WiFi coverage at your place.

The only exception is if you have some stationary device, such as older security camera or doorbell, which does not know how to communicate with WiFi mesh and can't connect to Deco mesh or connection is unstable. These are usually devices that only support 2.4GHz, as their WiFi code was written way before WiFi 5 and WiFi mesh became common - and has not been updated since then.

For such devices, assuming they can get WiFi signal from VR600, the workaround would be to enable 2.4GHz on VR600 with SSID different from Deco mesh, and configure devices connect to VR600 with its SSID.

Other than that, there should be no reason to keep VR600 WiFi on.