I can't speak of Venezuela but Iran's sanctions were too long that Iran already found customers to sell oil to (including Europeans!). So in a way there is no need to sell any additional oil to Europe anymore, specially now that Iran is increasing pressure on the West for 43 years of sanctions.
In fact if any sanctions were to be lifted, there is a good chance that mostly Russia oil reaches European markets through Iran.
Hint: A little more than 2 months ago Greece seized an Iranian oil tanker and Iran responded by seizing 2 US oil tankers. The Iranian tanker was actually carrying Russian oil and was heading towards Western Europe.
Already Russians are transporting part of their oil in the stealth mode. Ship-to-ship transfers are taking place and Asian as well as European refineries are being used to produce finished products sourced primarily from the Urals crude. With prices remaining constantly above $100 per barrel, there will be a lot of demand for cheaper crude (not just from the third world nations, but also from the developed ones). Iran is definitely going to be one of the beneficiaries and as mentioned above, I foresee a large part of Russian crude supplies rerouted through Iran in the near future.