MPG and PG appear to be the same compound, with both having the CAS number 57-55-6. The mono here implies it is not a dehydration "dimer" of PG, or it is the
monomer, meaning precursor unit used to make the dimer. For example, there is ethylene glycol, HO-CH2-CH2-OH, and there is diethylene glycol, which is the product of dehydrating two EG molecules to form diethylene glycol: HO-CH2-CH2-O-CH2-CH2-OH. One could call ethylene glycol monoethylene glycol. There is a dipropylene glycol (actually is a mixture of three compounds possible from dehydration):
Dipropylene glycol - Wikipedia. The prefix mono is more of an industrial term than a chemical name term, since there are di- and tri- glycols, as well as polyglycols used for all manner of solvents and plasticizers. I know, all very technical, but until now, I had not heard of MPG either.
So the bottom line is MPG and PG are one and the same compound, and as someone already said, make sure you use USP grade. Food grade is not considered pure enough to inhale.