Choosing values of resistive divider, versus pullup resistor value
I can't really see how there would be interplay between the reset output and Vcc input. It is an independent open-drain output and doesn't care about the voltage that is on that pin.

However, there is absolutely no reason to ever do what you are doing here. Reset monitors are there to have very predictable reset characteristics. When you use them in a way they are not designed, they may not be that predictable anymore. And why? There are many (hundreds) commodity reset monitors with every imaginable reset threshold. The current draw on Vcc by ADM6320, when it needs to pass through a high-value resistor like you are doing now, will cause a (variable) offset in the voltage on that node, and subsequently it will ruin any semblance of accurate reset/hold voltage thresholds. 

You would be much better off choosing a different part, for instance APX809, a bog standard supervisor chip. You can then use a discrete watchdog ORing circuit to replicate the functionality of ADM6320.

I see that in your schematic, the reset outputs goes to an enable pin on an LDO regulator. I assume that LDO in turn turns on a microcontroller? If so, this is a bad design practice. The function of supervisor circuits is to keep whatever chips it controls in a reset state until your power lines are OK. They should tie in directly to reset lines on these chips, not to the power converters that power these chips. If you do the latter, a malfunction of the LDO or power circuitry in general may evoke erratic behaviour of your microcontroller which is exactly what you are trying to avoid with this supervisor.