Household central built in ducted vacuum cleaner system 50mm PVC ducting pipe end cap or blank, for stopping the ducting, either temporarily or permanent.

It fits over the external diameter of the pipe.

If you want to blank off a sweeping T for instance, you'd cut a 50mm length of pipe, put this cap on one end, and put the other end into the internal flanged end of the  sweeping T fitting

Vacuum pipe is a different class to retic pipe, plumbing pipe or waste pipe, so they won't fit together for safety reasons.