I don't drink tea so not sure why I joined the TSG....maybe I misunderstood and thought it was the Toast and ....
Anyway, back to science..
So the inner sleeve of a Vacuum flask is usually silvered glass. There is then a vacuum between this inner and the outer skin.
Whether this is simply an insulated barrier or an actual vacuum, I am not 100% certain but, assuming the latter, we are still left with the fact that the cylinder itself is not fully enclosed and there is never a total vacuum.
There is an opening that is utilized to fill/empty the flask (generally at least 50% of the cylinder diameter) that is NOT sealed by vacuum, this is why if you were to pour a hot (or cold) liquid into said flask and left a gap, such that the container was not absolutely full, then the liquid would, in time, cool down (or heat up).
Correct me if I am wrong, but the integrity of the flask must be dictated by the quality of the stopper, rather than the container itself and that would determine how well the flask would stand up to the pressures of diving?