Filter/vac issues...

On Sunday morning I noticed that my pool was cloudy. I checked the chlorine and noticed it was a tad low so I raised the FC level to shock levels. This morning the pool was even more cloudy. I ran the filter from 9 PM to 9 AM (a minimum of 12 hrs). I noticed quite a bit of dirt on the pool steps, which is definitely not normal, just 2 days ago it was spotless.

I checked out my filtration system and noticed that it was pulling water fine. The pressure gauge is reading less than 15, which is decent. I turned the skimmer valve completely off and the pressure gauge shot to zero. But, this is the odd thing, the vacuum is still vacuuming, but the pressure is zero... how is that possible?

I stuck a hose in the filter skimmer to push water back out towards the pool to see if there was some debris caught in the vac pipe, but aside from a few leaves, it seems clear.

I turned off the filter and opened my tank, I have relatively new cartridges, so I checked them and everything is fine. They aren't looking dirty. One thing to note, when I opened the tank there was no pressure, it basically popped right off. Normally I have to let the pressure out before I can take the tank off.

One thing to note: the returns are sending plenty of water with the main skimmer open, but as soon as I close that valve and just have the vacuum valve open the returns send zero water back to the pool, even though the vacuum is still sucking.

I'm stumped. Is there some obstruction in the vacuum pipe that's allowing some suction but not enough to get a proper pressure established? If that is the case, how would I clear that? Could I use a plumbers snake?

Help!