If you're dealing with single molecule cracks and extreme vacuum, even vacuum grease is going to offgas and contaminate things.

Hydrogen is EXTREMELY small, the temps the poster is talking about are very very low, and XHV/UHV is extremely low pressure vacuum.

Like 'single molecules we don't want rattling around is a problem', 'more vacuum than outer space vacuum', and 'oxygen long ago turned into a solid' temperatures.

[https://en.wikipedia.org/wiki/Vacuum_grease]