Unfortunately you can't send custom quotes via Vrbo. This is a limitation on Vrbo's end. There are three options to offer discounts / special offers for Vrbo guests:
Guests check out at regular price on Vrbo, and you as the vacation rental managers can update / modify the reservation afterwards on Hostfully PMP to reflect the special offers. For more details, check out this article on Modifying a Reservation.
Vacation rental managers can credit a direct booking to Vrbo. That way, you can make sure the discount / special offer is offered in the first place. Please read more in this article on how to attribute a booking to HomeAway/VRBO.
While Vrbo takes a commission for Vrbo bookings, they also reward properties with placement in searches if they have more Vrbo bookings, so there is a pro and con to giving credit to Vrbo).
Changing the pricing in Hostfully and hope that nobody else books it before that guest!
The first suggestion is recommended since the whole booking process stays in Vrbo flow and you don't have to take risk of having another reservation with wrong price.
Please let us know if there is any question via pop-up chat on the page's bottom, or via email to email@example.com.