Get the "spray n wash" pre treater out of your laundry room and soak the stain real good with it. Then, using your wet/dry shop vac (remove the paper filter first or you'll ruin it) dump LOTS of HOT water on the stain area and suck it out again. Keep flushing it, and you should get there eventually. Make sure you get as much water out as you can, and keep flushing it so as not to leave any soap-cleanser-commercial cleaning product residue behind to collect dirt later.
Unfortunately, many cough syrups contain red dye #??? and so the stain is more like a permanent thing, but this will take care of some of the issue.
If it's still really bad, you can very carefully try some Cascade dishwasher soap with a toothbrush and some hot water, but do this carefully as it can bleach out the carpet fibers too much and you will have a whole different problem on your hands.
If this still doesn't do the trick, rearrange your furniture...