HMC851 Vr min value

I had a look in our archives and was not able to find any data with the VR voltage taken below -1.2V. I see a plot in the datasheet showing output differential voltage vs VR and temperature. Notice that the cold trace is heading south at a fairly steep rate and the hot trace is also starting to deviate. That would concern me a bit, even if you don't plan to use the part at cold or hot. 

So I'm not saying that you can't do this but I would say that we don't recommend it based on the little bit of information that we have. 

Would you consider instead setting VR in-range and putting some attenuation at the output to get the level that you want? 

I've not heard back from you and will assume that you do not need additional assistance.