Context: How is a Braintree vault payment processed on the Magento 2 server side?
Step 1. Magento\Checkout\Model\PaymentInformationManagement::savePaymentInformationAndPlaceOrder()

$paymentMethod->_data:

Step 2. Magento\Checkout\Model\PaymentInformationManagement::savePaymentInformation()
Step 3. Magento\Quote\Model\PaymentMethodManagement::set()

Step 4. Magento\Quote\Model\Quote\Payment::importData()

Step 5. (the Vault case)
How are the payment data assigned to the Vault payment method instance on an order placement?