The initial solution I posted ended up being only a partial solution. A big thanks to Marius for answering this question. With his help and some information from this question, I was able to piece together a working solution.
Summary
I have a single tax rule that encompasses all product & customer tax classes. This is essentially hijacking the tax calculation process to check if the customer is in Florida, and if so, do they have the Tax VAT field filled in?
Code
I extended the Tax Calculation rules by creating a file /app/code/local/Mage/Tax/Model/Calculation.php
Here is the final (working) code block:
public function calcTaxAmount($price, $taxRate, $priceIncludeTax = false, $round = true)
{
$cust = $this->getCustomer();
$custId = $cust->getId();
$vat = $cust->getData('taxvat');
$custAddress = Mage::getModel('customer/address')->load($cust->default_shipping);
$custState = $custAddress->getData('region');
$customerLoggedIn = Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress()->getRegion();
$taxRate = $taxRate / 100;
if ($priceIncludeTax) {
$amount = $price * (1 - 1 / (1 + $taxRate));
} else if (strtoupper($custState) === 'FLORIDA' || strtoupper($customerLoggedIn) === 'FLORIDA') {
if ($vat) {
$amount = $price;
} else {
$amount = $price * $taxRate;
}
} else {
$amount = 0;
}
if ($round) {
return $this->round($amount);
}
return $amount;
}