Bird of Fire:
Where does it say in the Popol Vuh that Vucub Caquix has anything to do with volcanoes? He was the false sun of the last creation, and was taken down by the hero twins of that saga. This is the bird at Izapa--not volcanic imagery. You have absolutely nothing to back this up except your fertile imagination. The two animals that have fire imagery in Meso-america are the butterfly and the xiucoatl (the fire serpent, not the bird).
Answer: The Aztec Calendar Stone has two serpents that encircle the disk. In each fire segment of the serpent, there is a headless bird form. It believe this would indicate that a fire bird is an integral element of the fire serpent called xiucoatl.

Vucub Caquix may well be the false sun (the Aztec version of Quetzalcoatl, Xolotl), that fell into the Underworld) because it was the Ring Nebula in the constellation Lyra (the jewel in the beak of Vucub Caquix) which seems to be part of the trajectory from a super-nova or a giant blue star Most cultures call it the "rainbow serpent" but it was also known as a bird with an iridescent rainbow tail, that carried a flaming ball on its back that it dropped into the ocean sea. The ball on the back of a bird with two heads is also found on a capstone at Ek Balaam.