Sorry about the delay.
There are good reasons why you cannot use such a combination:
-routing your VPN traffic via another proxy might compromise it, thus rendering your VPN useless
-doing it the other way around won't work, as the Adblock proxy then would need to be behind the VPN one – which you cannot establish on your device
However if Adblock Plus would be working by different means that could possibly work, but at the moment we have no plans for developing it.
Thank you for your interest.
All the best,