generally, annealing in vacuum (in comparison with annealing under inert atmosphere) has the advantage that the heating and cooling can be better controlled, which is favourable with regards to the dimensions (shape and dimensional stability) of the products.F urthermore, after annealing in vacuum, it is possible to cool with overpressure nitrogen or argon so that a faster cooling compared to the annealing under Argon can be realised. On the contrary, you might observe the effect of external pressure of inert gas on the higher stability of your material. Please also keep in mind that some compounds show poor stability in vacuum.