I’ve looked into putting in smart thermostats but I think tenants will call when they can’t figure them out.

In my county a tenant must have both thermostatic control of their space (meaning the landlady can't have the thermostat in her part of the house).

But with the VRBO renters, it doesn't appear that I have that sort of flexibility to write in the utility contingency.I'm considering putting in a NEST thermostat in the house and a remote pool pump monitor so I can keep an eye on where renters have set the temps?  

What you could do is have it set to a particular temperature with a lock box over the thermostat.  

I would plan to pay for majority of upgrades with a line of credit.I have a few additional reasons beyond monthly cash savings to sub-meter the electric: 1) There is one thermostat in the house, controlled only by tenants in the largest unit.

It is run with wall thermostats. 

I'm just about to start my first house hack duplex rehab project here are ideas I'm considering:-high efficiency toilet system-ceiling fans in bedrooms and main rooms-electronic keypad door locks-programmable thermostat-programmable electric bathroom baseboard heater

@Aaron Klatt correct me if I'm wrong, but there will be separate thermostats that are controlled by the furnaces?

Depends on the insulation, size of the house, how you set the thermostat.

Regarding higher costs for tenants, we inform them they can turn down the thermostat(s) in rooms that don't need heating during the day (bedrooms) and those savings can help reduce total heating costs to tenant.If you do not use the furnaces in basement, be sure to run a heater or two down there to make sure pipes don't freeze--winter is only a few months away.