Presently two methods of integration are supported with Android Pay on Vantiv eCommerce.
1) Using eProtect where Google calls the eProtect service directly and returns a paypageRegistrationId to be used by Vantiv for autorization / sale transactions
2) A second method where the merchant decrypts the payment information and submits a LitleXML transaction with the network token submitted as the "card"
Will both of these methods work will Android Pay Mobile Web?
Since I originally asked this question, Android Pay Mobile web has been delayed, so I suspect it will be a little longer before we have specific details. Early indications however are that the answer is "yes" - the same two integration methods with Vantiv are expected to be available for Android Pay In-App and Android Pay Mobile. It is expected that for Vantiv merchants support Android Pay "in-app" adopting mobile web functionality should be straightforward.
It is expected that Google's implementation will be based on the W3C draft standard Payment Request API detailed here - Payment Request API .