It looks to me like this is a covariance vs. contravariance issue. Basically, an IQueryable<Vacancies> is not a sub-type of IQueryable<IFilteredEntities>, even though Vacancies implements IFilteredEntities. Thus, the line with the cast is causing a runtime error. So rather than doing the cast try this instead:
IEnumerable<IFilteredEntities> selectedListings =
Repository.Instance._entities.Vacancies.AsQueryable()
.OfType<IFilteredEntities>();
What this will do is project each element of the collection to an IFilteredEntities type.
Another option is to rewrite your filter methods so they use generics, like this:
public static IEnumerable<T> Filter<T>(
IEnumerable<T> collection, IDictionary<string, string> filterParams)
where T : IFilteredEntities
{
...
}
This would then allow you to pass in a collection containing any type that derives from IFilteredEntities and get back a collection of the same type. And if you're using C# 3, you don't even have to specify the type parameter if it can be implicitly determined by the compiler.