Just bought the house. I have an attic with multiple types of insulation. I have 3" of the original cellulose insulation, followed by 6 to 9" of some kind of insulation that looks like cotton in a pink plastic bag (it's about 25' rolls), followed by 6" of unfaced fiberglass batts perpendicular to the "cotton" rolls. Does the pink plastic on the cotton rolls function as a vapor barrier? Does it breath? Do I have or need a vapor barrier? I have a ridge vent, soffit vents and a gable fan ventilating the attic. (FYI: I live it St. Louis MO.)

With a plastic material in the layers, I'd say yes, you have a vapor retarder. I would also say it's not in the right place if it's sandwiched between layers of insulation. I imagine it gets cold in Missouri in the winter, so the plastic vapor retarder should be as close to the warm in winter side of the assembly as possible.

I've seen that bag type insulation in the big box stores around here. They mislead people in thinking that that will be an adequate vapor barrier, but the gap between the bags when laid side by side isn't sealed and because of that, have lost any vapor protection that you think you are getting.

My advise is to go in the attic and look for any evidence of water damage and if there is none, then leave it alone for now. Go back up toward the end of winter next year and see if there is any frost or moisture buildup up there anywhere.

No the plastic bag that contains the cotton looking insulation is not a vapor barrier. Its there to simply hold the stuff together.
Ideally yes you should have a vapor barrier. But it should be located next to the ceiling area of the insulation layer. At this point I would just use a vapor non-permable paint on the ceiling.

Is my main concern being water condensing under the pink plastic bag insulation? Or is it warm water vapor getting through all of the insulation and condensing somewhere in my cold attic? It seems that with 6" of fiberglass insulation over the 9" pink plastic stuff, that condensation under the bottom of the pink stuff won't happen because it is protected by 15+ inches of insulation above it, keeping it from being cold enough for condensation on the bottom.

The plastic sheet is definitely a vapor retarder no matter how ineffective it is in the overall scheme. The fact that it is not in the proper position within that insulation system could be a cause for concern. Provided that the insulation above it keeps it warm enough to remain above the dew point in any given temperature/relative humidity condition, it will not be a problem. Any relativelty impermiable material can act as a vapor retarder. Even hefty garbage bags used for storage in an attic can act as a vapor retarder and will condense moisture under them in the right conditions. I've even seen condensation on the bottoms of plastic storage bins that were placed close together where a vapor retarder was not used. I would do as suggested. In the coldest part of winter, lift some of the layers and have a look. Check at various levels throughout the system to see if there is any point where moisture is present.

That's very interesting. I spoke with the previous owner and he said he never had a moisture problem with that pink stuff. He was told that the pink plastic cover was vapor permeable and that it would simply pass through it. Has anyone ever hear of this?

I also have a 24" wide catwalk down the center of the attic floor made out of 3/4" plywood. It is covered with 12" of removable fiberglass batts. Would something this wide and solid act as a vapor barrier or would the moisture just go around it?

Although I've never seen it happen personally, I've heard of the glue layers in plywood acting as a vapor retarder. 24 inches is certainly wide enough. There are a lot of variables. though. If you have 12" of insulation on top of the cat walk, my guess is that it won't be a problem. Like was posted before, check it in the cold season. Then you'll know for sure.