And since you know (roughly) what Vbe is (and that's NOT the voltage between the "base" and emitter terminals of this device, because you don't have access the the base directly), when Vce is less than this, the collector junction is forward biased.
These devices are not intended for analog use, their specifications leave unspecified what will happen where the input terminal has a voltage on it between the max OFF voltage and the min ON voltage. What happens between those voltages can probably be calculated, but I'm not sure with your level of experience that you're up to it. The large variation in the actual values of the internal resistors also means there is a significant amount of uncertainty in the current drawn at the "base" terminal (but not the voltage required). All up, it is better to use the values from the datasheet and indicate when the outputs are guaranteed to be in one of two states, and the voltage range where the outputs are uncertain.