@Lesauchet 

Hey

The Deco and the VR can work together OK, you have not wasted your money.

However I would advise disabling the WiFi on the VR600 and using only the Deco for WiFi.  You technically don't have to do that, however the Deco will allow you to roam around without disconnecting, this doesn't work with the VR600 only the Deco.  Therefore best turning off the VR600 WiFI

Once that is done, plug the first deco into any of the LAN ports on the VR600 and open the DECO app on your phone

Any hassle just drop a message back

@Lesauchet 

You can use VR600 as a router and Deco mesh for WiFi coverage. This would be similar setup to mine, where I have ISP provided cable modem/router to which I attached Deco mesh for good in house WiFi coverage.

For that:

1. You will need to turn off WiFi in VR600. If you don't know, how, you'll need to check router documentation.

2. You will need to configure Deco mesh to run in Access Point mode. See the guide to configure the Deco E4R as an Access Point: https://www.tp-link.com/support/faq/1842/

I had no problems configuring Deco with my router, and you weren't specific what problems did you encounter with Deco initial configuration. See the following troubleshooting guide for help with Deco setup: https://www.tp-link.com/support/faq/2239/