This is already doable, and always has been.
When using the Vrbo API, Vrbo passes the guest's credit card data in when the booking is made. OwnerRez automatically takes the first payment, typically 50% (sometimes 100% if it's a last-minute booking), schedules the second payment at the appropriate time, and sets up the security deposit hold.
But, that second payment is a "smart" payment - it'll only collect whatever balance is due when it fires. So, if the guest mails in a check before then, and you manually record that as an offline payment, the balance will be $0 and no second payment will occur.
Or, if you are confident that you'll be getting payment some other way, you can simply delete the second, scheduled payment from the Transactions tab of that Booking. That leaves it entirely on you to make sure you get that second payment.
Of course, manual payments can be anything, whatever you are willing to accept from the guest - all that matters to OwnerRez is that you record the payment amount you received in Transactions for the Booking.
Vrbo considers API bookings to be handled entirely by OwnerRez - once the booking has been created, you can make any changes you like to it, and Vrbo won't mind.
