Вы можете попробовать LINQ query-style join:
incidentVm.LstSpecialCategories = (from aispc in db.TBL_AssocIncidentSpecialCat
join spc in db.TBL_SpecialCategories
on aispc.SpecialCategoriesId equals lspc.Id
where aispc.IncidentId == incidentVm.ID
select lspc.SpecialCategory).ToList();