Vagionia Beach sits on the quieter north side of Poros, away from the easiest town beaches. The appeal is clear water, a more sheltered feel, and the sense that you have made at least a little effort to get beyond the harbour.
Facilities can be limited compared with the busier beaches, so bring what you need and check transport if you are not driving. It is a good choice when Kanali feels too convenient and you want a swim that feels more island-like.