I've been using my roof run off water for years for CPs, no problems at all.

There are two contamination sources I can think of. First one is your water may have salt in it, if you live near the coast. The second is that your roof may have been treated with chemical.
But the thing that gets me is that if you use the same water with all of your CP and your VFT is okay but your Nepenthes is dead. There are maybe more to it than just water because Nepenthes is most forgiving when it deal with watering comparing to other CP plants

Should have been more clear, the pitchers are not Nepenthes, they are different kinds of Sarracenia.

Yeah, I think it is your water. What I think happen is that the impurity sink to the bottom of your container. It store in the lower part of your media. Sarracenia roots are about 3 times longer that VFT for that it took the hit sooner than the VFT.
I think you should remove the VFT from your media and start water them with bottle water instead.
You may want you look in to reverse osmosis systems

Do NOT use bottled water unless it says DISTILLED on the label.

chill out dude as long as PPM is lower than 100.
Get a TDS meter