# models.py

class VacancyManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().all()

    def get_vacancy(self):
        print('yo')
        return self.vacancy_set.all()


class Project(models.Model):
    name = models.CharField('Название', max_length=100)
    description = models.CharField('Короткое описание', max_length=100)

    def __str__(self):
        return self.name

    objects = models.Manager()
    vacancy = VacancyManager()

# views.py
class ProjectDetailView(FormMixin, DetailView):
    template_name = 'projects/project.html'
    model = Project
    form_class = LeaveProjectForm

    def get_context_data(self, **context):
        print(self.object.vacancy)

# trace
    raise AttributeError("Manager isn't accessible via %s instances" % cls.__name__)
AttributeError: Manager isn't accessible via Project instances