Can you lose money in Vanguard money market settlement fund?

Possible but very unlikely. Money market funds invest in high grade liquid assets and the primary goal is not to lose money. I have not been able to find an example of a Vanguard money market fund ever "breaking the buck" and having the value of a share go below a dollar. It is possible that this could happen in the event of a large scale financial collapse, but even then I would call it possible rather than likely.