To find the positioning reference plane of the CNC vertical lathe workpiece, you can follow the following steps:

1. Understand the design requirements of the workpiece: First, understand the design requirements and processing requirements of the workpiece. This will help determine the datum plane suitable for positioning the workpiece and the corresponding measurement and positioning methods.
2. Check the process drawings and specifications: Check the process drawings and specifications carefully, especially the features, dimensions and tolerances related to the positioning of the workpiece. This information can often guide you to a suitable datum for positioning the workpiece.
3. Analyze the shape and characteristics of the workpiece: Analyze possible positioning datum planes based on the shape and characteristics of the workpiece. Common datum planes include planes, cylinders, prisms, etc. According to the geometry and design requirements of the workpiece, select the most appropriate datum plane for positioning.
4. Use measuring tools and equipment: Use measuring tools and equipment, such as calipers, micrometers, vernier calipers, height gauges, projectors, etc., to measure and inspect workpieces. This will help you identify planes or features on the workpiece that may be suitable as positioning datums.
5. Consider process requirements and processing sequence: When determining the workpiece positioning datum plane, you also need to consider process requirements and processing sequence. Ensure that the selected reference surface can meet the continuity and accuracy between processes to achieve stable positioning and uniform processing of the workpiece throughout the processing process.
6. Verify and adjust: After selecting the workpiece positioning datum plane, verify and adjust it. Use appropriate measuring tools and methods to check the positioning accuracy of the workpiece and make necessary adjustments to ensure that the workpiece matches the machine tool coordinate system accurately.

The above steps can help you find the positioning datum plane of the CNC vertical lathe workpiece. Please note that in specific operations, appropriate fixtures, fixture components and positioning methods should also be used in combination with the requirements of the machine tool and process. Depending on the specific situation, it may be necessary to consult professionals or relevant technical information for more detailed guidance.