Since hats are made out of a wide range of materials, you'll need to match the hat to the correct style of vinyl.
That said, thermoflex works great on cotton hats... but you will see the seem and stitching through the vinyl on a 6 panel hat. You can avoid this by using a less popular 5 panel hat. The long term durability is probably better than t-shirts in most cases (as hats usually don't get washed!).