As I said in my comments, I cannot see exactly what you are trying to do but from what I gather I would try something like:
if(! lstRubriqueinter.Any(x => x.RubriqueId == vm.RubriqueId))
{
RubriquepointageVM rub = ServiceApplicatif.Save(vm);
// you may need to add this rub to the list at this point. You will have a better idea.
}
I haven't tested this code because I don't have much to go on really from your question.