How do you DEFEND against a VCT? Not only by blocking — by counter-fouring. When the attacker plays an open three, look for your own four: the attacker must answer it, and while they do, YOU take the junction their chain needs. Your move as WHITE: black's three here already stares at both columns, so a plain block loses to the 4-3 either way — find the counter-four, a move that blocks the three AND makes a four of your own.
Finish this step to move on.