DNS privacy and VPN are not dependent on each other. They can certainly be combined, but they serve slightly different purposes depending on the level of privacy you want.
Privacy DNS (like Decloudus) will ensure your DNS traffic is encrypted. It will block ads, trackers, etc. Decloudus also emphasizes that there are no logs, so there is no way Decloudus can associate your DNS queries to you. That’s a must-have-requirement for any DNS provider I use.
As others noted, a VPN will hide all of your traffic from ISP. When using Decloudus, your ISP will not see your DNS traffic, but the ISP will still know the IPs you are connecting to. For example, your ISP will not see that visited forums.puri.sm URL, BUT they will see you connected to IP address 22.214.171.124, which then maybe used to know what site your actually visited (through reverse DNS lookups… but who knows if ISPs do that or not)
I have devices that just use Decloudus DNS and other devices that use both Decloudus and a VPN. For example, I have gaming console, kids tablet, etc with internet traffic that I don’t care if the ISP knows about. But, I do want my DNS traffic to be private and I do want to block ads, trackers, and such. So using Decloudus DNS is enough for my privacy needs there.
I have other devices where not only I want to have DNS privacy, block google, ads, trackers, etc… but I also don’t want the ISP to know what IPs I am connecting to. So I route the traffic for these devices to VPN also.
So depending on the specific privacy needs you have, Decloudus can be good enough on its own or add a VPN to the mix for even more privacy from your ISP.