What is the vapor pressure of water at 750C?

There's not really enough information to answer your question.

I assume you're asking what the pressure of water vapor would be at 750C, which would require the use of the universal gas law.

PV = nRT, where P is the pressure of the gas, V is the volume, n is the number of mols of gas you have, R is the universal gas constant, and T is the temperature.

It can be rearranged to be P = (nRT) / V

And you could find each variable independently to find the pressure.

R is 8.314, as it always is (being a universal constant)

n can be found using n=m/18.02, where m is the mass of gas you're measureing

V is the volume of whatever container you're using, if it's a geometric shape you can just measure it

T is 750 + 273.15 = 1023.15, because temperature is always in kelvin and it was given in celecius in the question.