The only research available (http://www.uvm.edu/~pmrc/vacsap.pdf) has shown that there is no trend of decreasing sugar content with increasing vacuum level....at least up to 25" Hg. There really isn't a whole lot of possible physiological reason for there to be a difference. I could see this being a possibility during a long extended thaw, but for any single, relatively normal sap run, there shouldn't be any significant difference.

vac vs sugar.JPG