moles of H2 present = 2/18 *4.09 = 0.45 mol
moles of O2 present = 16/18 *4.09 = 3.59 mol
This is wrong - use just PV=nRT to caculate separately amount of hydrogen and oxygen. No idea what and why you tried here.
Idea of doing the calculation by finding limiting reagent is OK.
so how do i find the vapour pressure since H2O (g) is not ideal and i cant use PV=nRT
First of all - water vapor is close enough to ideal at this temperature.
Then at 25 deg C H
2O is not gas.
Then I have no idea what you are expected to do - could be you are to ignore presence of water vapor. In general this is wrong and you should check saturated vapor pressure in tables, but this thing is sometimes ignored on the HS level.