How could I get one of them to agree to sign an agreement to vacate the property?   

Our lender says that's normal because its a vacation home. 

That gives the agent a stronger offer and you have a greater chance of winning the home with a loan commitment versus a Pre-approval letter.As of this morning rates are down on all programs for Primary homes, Second/Vacation homes, and investment homes. 

an advantage might be something like:-used to live there and know it well-it's close by-have friends and family there-like to vacation there / want to own a second home thereif i were you, i would pick a market you can go to in person often, or at least in the beginning when you can look at properties and set up your team. 

It's better to find a tenant for a discount than to let it sit empty.We also aim to complete turnovers within one week of the tenant vacating.

If business is slow, plan a vacation.

I gave a tutorial on knowing your enemy for my location How to Comp Vacation Rental Income Projections in Coastal North Carolina

How could I get one of them to agree to sign an agreement to vacate the property?

pick a market this way.a place you used to live in, or went to college in, or have family, or like to vacation in, or know really well.also: consider investing closer to where you live. 

(Max vacated rents - your monthly mortgage payment) = +/- net cashflow.