why models are not loading in autoload

Make sure your models are in the application/models folder and start with a capital letter.
When loading a model with $this->load->model( ... ) don't use a capital letter.
So, it's perfectly ok to load the Vacancy_model.php with $this->load->model('vacancy_model');
To refer to methods (functions) inside the model: $this->vacancy_model->function_name();