That's how things go sometimes. It doesn't take much of an air leak to mess up the vacuum suction.

Ok, if this can help anyone I am putting my case in. I have had the low pressure vacuum line issue 2 times now. The first time it had lot of suction until you plugged in the vacuum, then it get very weak. the vacuum would not run. After lots of checking I decided it was a clogged line. I tried the shop-vac to no avail. So then I took the screws out of the vacuum switch valve and pulled the valve out. I then took a piece of plastic bag about 4x4 and tied a string to it. I put it in the switch valve down the vacuum pipe. I then hooked up the shop-vac to the hose in the pool and tuned it on. With a little effort it pulled the line thru the clogged spot and out to the shop-vac. Bag on the valve side I tied a big piece of plastic bag on the string. (plastic big enough to fill up the pipe) I then pulled on the other end and pulled the fat piece of plastic thru the pipe until it hit the clogged area. It would not budge any farther. So I took my water hose wrapped the end with a plastic bag and shoved it down to valve side vacuum pipe. I turned on the water and pulled on the string. The water pressure and the pulling dislodged the debris and I was able to get it out. Turned out to be a couple 1inch rocks and a round pool thermometer. No more of those allowed in my pool.!! All this sounds a lot hard and longer than it really took. I did have help.
The second time took a lot longer to figure out the problem, turned out the be the o-ring on the pool/Jacuzzi valve. Everything worked fine until I would try to use the vacuum, then all the pressure would fall off and the pump would lose prime and the filter tank would fill up with air as well as air bubbles in the pool. Replace the o-ring and worked great. Hope this helps someone.