I called vise today. I probably can't do the long conversation justice, but he said that the number one cause of bubbles is moisture. That even a drop of water will cause it to bubble.
Also the plug cures rapidly (obviously), and you need to pour once you feel it is warm. If it is too thick when you pour, air bubbles from the pour can get trapped and won't rise to the top because the plug is too thick. He told me to pour down the stirring stick to minimize bubbles during the pour.
Finally, he said that the cups they give you can be reused. That after the plug cures, you can pull it right out of the cup. If that is true, that is nice.