I would imagine it would be around 47 psi. if 1150 kgs. is the fully laden weight (MTPLM) but If you have a look on tyre wall it should tell you what the maximum tyre pressure is.
To get it right you need the load index and the max psi of your new tyres (written on the sidewall).
If for instance, the load index is 94 and the max psi is 65, the tyre can take a max load of 670kg at 65 psi.
Your individual tyre load is 575kg, so if you divide 65psi by 670kg you get 0.0970149. Multiply that by your load of 575kg and you get 55.7836 - near enough 56psi. So 56psi would be your tyre pressure in that instance.
Its worth getting it right! Under inflated tyres can cause blowouts.
I have the same size tyres, 8 ply rating with a load rating of 97 and the max caravan weight is 1100kg, I run mine at 52psi as this works out via the formular
The Formula will always give a higher than required pressure, pretty accurate if close to the maximum pressure but becoming 6-8psi to much as you get further away.
In this case, there are two load rating for the 175R13C tyre, for 1150kg and the 97 rated tyre the correct pressure is 48psi, for the 94 rated it's 45psi
PS, for 1100 kg Vectrolot, you should be running 45psi, 52 will be giving your van an unnecessary hard time!