The pressure at the vacuum relief valve is largely going to be determined by the return line from the panels to the pool. Since it is all downhill, there should be very little pressure. In fact, in a properly designed system, you will probably need to add a restrictor to increase the return pressure enough to close the vacuum valve. If the pressure at your vacuum valve is 5 to 10 psi, that would indicate that the plumbing from the panels back to the pool is too small.
Assuming 15 foot panel height, that would mean return head of 6.5 psi at 0 psi at the vacuum valve. That would be equivalent to 48 gpm going through about 300 feet of straight 2-inch PVC pipe. So, I wouldn't think that your pressure at your vacuum valve would be as high as 5 to 10 psi.
Fafco installation manual shows, the rated operating pressure at 28°C (82.4 F) = 2.40 bar (34.8 psi), and at 90°C (194 F) = 0.34 bar (4.93 psi). This shows that the pressure rating drops off significantly as the temperature goes up.
There will be times, such as upon startup, that the panels will be well above 120 degrees F. It is my experience that most equipment does not do well at ay pressure above 1/2 of its rated operating pressure. And, this is for Fafco panels, which are probably better quality than some of the other brands out there.
I think that the best way is to measure the flow rate with a flow-meter to make sure that it is within the specified range and then to increase the return resistance until the vacuum valve closes. This way, you get the required flow at the minimum operating pressure.