Vacuum cannot have a temperature, by the definition you are using. This makes it useless for certain tasks, such as determining the temperature an object within a vacuum will stabilize at over time. Thus, we use a different definition of temperature - because saying 'the temperature an object will stabilize at' every time instead of 'temperature' is really wordy.
You seem to agree with me since you explain how 'temperature' may be assigned to vacuum as a fudge. This is a dangerous thing to do as equilibrium temperature does not depend on the vacuum itself but on any radiations present.
