when the compound is at the pressure of its vapour pressure, the equilibrium is constant.
since u want a headspace for the vapour, total conversion of gas to liquid is not neccessary. in that case the pressure required should be the vapour pressure at 25 degrees celsius.
im not sure about the exact pressure, but judging by the values at 21.1 degrees celsius (approx 70 degrees farenheit) and at 37.7 degrees celsius (approx 100 degrees farenheit), the value should be somewhere between 61.3 and 106.0 psig.
1 psi = 689.5 Pa, and psig means that the value given is a measure of how much pressure above atmospheric pressure. in other words, u add in 10 kPa to the value to get the total pressure.
so the vapour pressure would be between 42 300 + 10 000 Pa and 73 100 + 10 000 Pa or 52 300 < x < 83 100 Pa.
it should be more closer to 52 300 seeing that 21.1 celsius is a closer temperature.
i cant find the exact value because all the gas laws require a constant volume, which is of course not present in your case.
should the increment of vapour pressure and temperature be linear, then u can treat it as an ideal gas thus using the equation
P1/T1 = P2/T2