ad blocker w/ vpn, or vpn with ad blocker?
Most VPN based ad blocker Blokada don't work like traditional VPN systems, instead the VPN server is located on the same Android device. Outside of your device the traffic is just normal Internet traffic exactly as it would be without VPN based ad blocker.
The VPN based ad blocker uses the VPN system only for redirecting the traffic into the app for being able to filter the traffic. On unrooted devices the VPN system is the only way to get access to the traffic of other apps, therefore the Android VPN-client is used.
On regular VPN systems the VPN server (endpoint) is located on a server somewhere on the Internet. If the VPN is active all traffic that leaves your device is encapsulated (protected + encrypted) in a VPN data connection that is sent to the VPN server and there the data is unpacked and sent to the original server. Therefore the traffic that leaves your device is totally different and because of the additional server in-between the latency can increase depending on where the VPN server is located.
Of course it is possible to add ad blocking features to a traditional VPN.