Product review (4 stars): Works - all you need is some scissors.
As for function, I would hate to see what my humidifier would look like without having a demineralization pad in it. They are always disgusting and cruddy when I throw them out (about every 2 weeks) and that's using filtered water. My scale buildup in the heating element/resevoir is manageable and comes right off after a 20 minute vinegar soak. So I think they're doing their job.

Yes, they are "too big" for the Vicks humidifier. I am, however, quite sure that everyone on this site owns a pair of scissors. Just cut them down, it's really not a big deal. They are literally twice as much as you get of the official Vicks/Kaz brand product for the same price. So if you value your money, buy these and trim them. If you have more money than time (as in 30 seconds to cut a rough cirle) buy the brand name and you will obviously not have to deal with trimming. For what it's worth, my Vicks humidifier came with the original pad just stuffed into the heating element/resevoir all willy nilly, which didn't hurt it at all as far as I can tell, and when I change them, even using the "right" ones from Vicks, usually by the time I take it out it's all scrunched up and not a perfect little circle of pad. It's really not going to matter if you can't get it cut into a perfect circle that is exactly the right size.

PS: These can't  "melt" like some reviews have mentioned. What happens is that if you have very hard water and you DON'T change your mineral pad or clean out the unit enough, the minerals in the pad bond to the minerals building up on your heating element, and create a big mess. If this happens, you can usually do vinegar soaks for a few days (soaking for 8 hours or overnight and then changing it, for about 3 or 4 soaks worth) and get all of the buildup and the remains of the pad out. Clean the unit once a week like the instructions state and inspect your pads and you'll bypass this headache - you'll know when it's time to replace from the looks of it.