American Holidays operate packages from both Belfast and Dublin, and also tailor make packages also.
The vast majority of customers from NI use VS from LGW connecting with FlyBE. I believe that American Holidays have an excellent contract with VS this year, so Dublin originating passengers may also be filtered through Gatwick onto VS flights from there. Don't believe that VS operate direct from DUB.
While the VS option may work out the cheapest with the current deal, American Holidays will still route on the direct EI and potentially CO from BFS, as tailor made options.