To connect a 1 1/2" PVC pipe to a 2 1/2" vacuum hose, you would need a reducer bushing with a 1 1/2" slip (glue) fitting on one end and a 2 1/2" threaded or barbed fitting on the other end. Here are some possible options:

1 1/2" PVC Slip x 2 1/2" Hose Barb Reducer Bushing 1 1/2" PVC Male Adapter x 2 1/2" Hose Barb 1 1/2" PVC Threaded Male Adapter x 2 1/2" Hose Barb

Make sure to select the type of fitting that matches the configuration of your vacuum hose (threaded or barbed). You may also need some PVC cement and/or hose clamps to secure the connections