I would definitely NOT recommend a venturi pump for the job that erubin will do. He will be flushing and blowing through the system with compressed air. The system will be PACKED with moisture. One of these will not pull a good enough vacuum for the SERIOUS evacuation that he will need.
I think it is worthwhile to point out here that the reason you are evacuating is to remove moisture. Moisture left in the system combines with the refrigerant to create an acid that will eat a hole in an aluminum evaporator in short order. R134 is even worse about this than is R12.