"Sealed in a vacuum" is an oxymoron, a contradiction in terms, much like "frozen with fire". If you seal a liquid in a flask containing nothing else but vacuum, then a part of the liquid will quickly evaporate and fill the flask with vapor, so it would no longer be a vacuum.
The said vapor will exert some pressure, depending on the temperature and the nature of the liquid. For example, with water at room temperature, it will be a lot lower than the normal atmospheric pressure. Indeed, the boiling point of water at that pressure will be a lot lower than the usual $100\;^\circ\rm C$. Guess what? It will be the same as ambient temperature. That's how equilibrium works.
Now what if we just keep the liquid at vacuum by pumping out the vapor as soon as it forms? Well, the simplified† answer is: the liquid will evaporate gradually, bit by bit, until nothing is left. There will be no more liquid, and consequently no more boiling point. That's what happens with the boiling point in a vacuum.
† In fact, it is more complicated. As the liquid evaporates, the rest of it gets progressively colder. At some point, it will get cold enough to freeze, and then you'll be dealing with ice. The molecules will still be leaving its surface, but slower and slower, and the complete sublimation might well take forever, or even longer. That's why the space rocks are still there after all these years.