Assuming von Baer's rule is followed, we see that the state of characters in Amphioxus is the primitive state. Your cladogram is complete, and, so we believe, correct. A is the vertebrate outgroup or sister group. Jaws appear within the vertebrates. Legs evolved too within vertebrates, and did so after jaws, or so we believe.
Legs define the Tetrapod clade.
Notice something rather very important. The frog that we added to the first table fit in nicely, neatly, with respect to more primitive characters. We mean that it was just like its sister group, the zebrafish, with respect to the cranium and jaws character states. The tetrapod shares these plesiomorphic characters with the sister group. Legs and jaws are derived within vertebrates, not shared by all of them.