Can I vacuum a/c system from low side only

Yes. You can pull a vacuum from one port.

The AC system is a continuous loop. There is a part known as an orifice tube which restricts the flow of refrigerant, but doesn't stop it. Drawing a vacuum from one port will still allow you to completely evacuate the system.

Note: If you are talking about removing either the Low or High side ports and not having it anymore, this isn't a good thing to do. The problem with this is, both ports are needed to properly diagnose issues with an AC system, as well as ensure it is properly filled when adding refrigerant.