Vacuum is the poor man's method of protecting protein drugs from oxidation. It's easily lost via leaks, and never eliminates all the oxygen. Pharma replaces oxygen with an inert gas, like nitrogen, so there's no vacuum pressure on the stopper trying to suck air in, and it completely eliminates oxygen.
Oxygen damages rHGH, and almost all peptides. Once a growth hormone molecule is oxidized it no longer works, unfolds, and causes aggregates to form by making more rHGH stick to it, reducing efficacy even further.
The TLDR of this study was that the presence of oxidized rHGH makes the entire vial more susceptible to temperature degradation than it would be otherwise.
Methionine is the amino acid in rHGH most likely to be damaged by oxygen.
"The thermal melting of r-hGH and its oxidized products is a highly cooperative process. Methionine oxidation leads to a thermal destabilization of the whole protein folding and is not just a local destabilization."
"Oxidation is one of the major issues in the development of a biotech product which can occur during various steps of the development, upstream / downstream processing, filling, storage and handling. Aggregation is also a major issue that can reduce the yield during production, induce immunogenicity, toxicity and loss of activity."