I assume that your plumbing system has a few "suctions" feeding the pump, one would be MAIN DRAIN, the other would be SKIMMER/OVERFLOW and last will be VACUUM all these 3 would have valves to isolate them or choose which one you want to use. This is what I have in my pool and I think this is a common set up.
When I vacuum, I will close the valve for SKIMMER ( in my pool its called OVERFLOW because I have a balancing tank, not a skimmer ) and will set the valve for the main drain at 50%. This way the vacuum gets concentrated but not to the point of being dangerous, since there is still MAIN DRAIN to allow water flow for the pump in case I clogged the vacuum hose.
40 foot vacuum hose is very long. Lots of power will be lost, most pumps are good at kicking water ( output ) but not at suction ( input ). Also to note if your hose comes in two piece where there is a connection since I think 33 feet is the standard single piece length, that connection is not very air tight if it floats on water surface. If you can sink that connection part, it will be good, your pump will not suck air. I use use lead weight to sink that connection, if it get submerged the vaccum power is good since it will suck water not air.
If you want to know the flow of your pump as in water flow, you will need a FLowmeter, it tells you Gallon Per hour or Liter per hour flow, installed on the pipe. If you want to know the power of the suction, you will need a Vacuum gauge that reads in Hg. I have a vacuum gauge and a pressure gauge installed on my Hayward 2HP impeller housing.
Long suction hose is a work burden for the pump, not just your vacuum hose, this will include all suction pipes that feed the pump. If you want concentrated suction power, you will need to shut all valves feeding the pump and only select VACUUM. However, this may cause lots of bubble formation visible in your pump strainer basket ( impeller housing ). These bubbles may not be air leak it may be simply too much suction resistance and the pump cavitates, hence you see those bubbles.
Too much cavitation will damage your impeller very fast.
You can simulate cavitation by closing all suction valves of the pump slowly up to the point the flow is so restricted, the air bubbles will formed within the water. Water is H2O remember.
I test my vacuum suction power using the palm of my hand, its painful...yep... but it tells me how much suction I have.