Unfortunately, that won't work. Oxygen masks only work up to a certain altitude, which is roughly the altitude where airliners cruise. At pressures lower than that, even pure oxygen isn't enough to sustain you.

You don't die instantaneously when exposed to vacuum, although it is pretty quick. I wonder if it would be possible to repressurize the tube (by blowing a big hole in it?) within 15 seconds or so.

The trouble with blowing a hole in the tube somewhere to re-pressurise is that whatever is inside the hole is hit by a wall of air travelling somewhere around the speed of sound. There are a LOT of problems with hyperloop not limited to someone walking up to the tube and shooting it with a .50 Desert Eagle, earthquakes, and the expansion and contraction of the tube as the sun rises and sets.

It's definitely the most unlikely of Elon Musk's plans.

The hyperlink won't be a vacuum. It's just a low air pressure chamber. The plan has never been to maintain a vacuum since that's too difficult at such large scales.

Hyperloop is planned to run at roughly 0.1% of sea level air pressure. For the purposes of a discussion about human survival in the event of pressurization failure, that's effectively a vacuum.