Vilnius is interesting and i believe it is one of the most likely of the Aer Lingus routes to be taken up.
My bets are not on Easyjet to do this though. The current connection from Gatwick to Vilnius is by AirBaltic via Riga, they even use a 757 sometimes to Riga as this is a popular route. Shuttle flights from Riga to Vilnius are very frequent. OK, AirBaltic might go direct again, but there is already Ryanair to Kaunas from various UK airports, and Star1 from Stansted, so it's not an obvious hole in the market. Easyjet dropped Berlin-Riga and Berlin-Tallinn a while ago, they don't seem to be that cozy with the Baltics. I don't believe Easyjet has an obvious cost advantage to either AirBaltic or Star1.