I have booked VRBO properties multiple times and always suggested to come to an agreement outside of VRBO to lower the fee. All worked out great, I have never paid the listed rate, usually 15-40% less in fact.
However, I have never had an owner request the same from me.
If that were the case, I would be cautious and would never pay anything before arrival. At the same time I would demand substantially lower rates.
As riced pointed out, the VRBO fees for homeowners are outrageous. But the listed prices already take that into account. So if the owner demands the listed rate but payment outside of VRBO, I would never agree but definitely negotiate.