Then it' a matter of liquid-vapour equilibrium.
The equilibrium pressure increases with the temperature. Optimally, you have the experimental curve or table that relates them for your compound. If not, maybe you have Antoine's parameters for your compound, and you can make more or less reasonable predictions. Or you know the heat of vaporization of your compound, and can extrapolate the vapour pressure at one temperature from an other temperature near enough.
This hold if the compound is far from its critical point. Near the critical point, the heat of vaporization drops a lot, so extrapolations are wrong and simple equations like Antoine inaccurate. This is a big interrogation since 5MPa is a near the critical temperature of many compounds.
You also need the partial pressure of any other gas that were in the vessel before the temperature rise.