When a player has bought Capitalism, actions that give +$ gain the Treasure type. This means that gaining such an action (including buying it) should trigger Falconer's condition and allow you to play the Falconer. However, in my recent game that did not happen. Both players bought Capitalism, but further gains of now-Action-Treasures such as Vassal did not provide an opportunity to play Falconer.
I tried to reproduce this but the game actually handles it correctly. I further looked at the game in your history that's probably the one that you're talking about here, and I couldn't see any point in the game where you actually had Falconer in hand and either player gained a Vassal (or any other capitalism-ed action). Could you be misremembering?