Measurement of current with hall effect sensor
You should measure Vout with respect to ground.
If you zoom out the picture there is a small voltmeter connected to Vout (see below). So, that's their suggested way to measure.



In the datasheet of another Hall split core sensor (HSTS08) the word "suspensibility" is written behind Vref. I think this is the magnetic susceptibility and the defiation from 2.5V is a measure of the magnetic susceptibility of the Hall sensor. But I cannot find documentation for it.  

You could try and play around by measure a significant current with the device and check how the reference voltage changes. Next, revert this significant current and see if the reference voltage returns to it's original value.

But best way of course is just contact them about Vref on https://poweruc.com/pages/contact.