You must remember that VRBO is simply a spot to advertise. All the properties are individually owned.
Your ability to get a last minute discount truely depends on how last minute your are talking and how hungry the property owner is.
As the owner of a beach house in Prince Edward Island, I can tell you that I have granted deep discounts on last minute deals (7 and under days out). At the same time, don't hold your breath for that deal in July or August.
On the flip side, I have negotiated discounts on numerous VRBO rentals, both because I was booking last minute or booking a low period.
As an example, I am going to Big White next week and negotiated 30% off the posted January value rate. The trick was I was going for a low period (5 nights midweek - 3rd week in Jan) and I was taking 2 units and was prepared to pay in pull at the time of booking.
Last year I booked a condo mid-week in March in North Conway. The place normally rents for $450 a night. It was a very nice palce. I paid $800 for 5 nights. I booked 1 week out for a slow week and it was a Sun to Frday rental. I asked for the best deal at 4 different units. I made clear that I was looking for a deal on a last minute mid-week rental. 2 of the 4 offered me good deals.
A landlord will offer a deep discount if they believe the alternative is being empty. The trick is to ask.
Hope this helps.