I'm certainly not familiar with pure nicotine; I use 100mg. When it goes "off," I don't know that it's the nicotine itself that has gone "off," but the carrier, PG or VG. I don't know if PG can go "off," but I'm pretty sure that VG can. But oxidation actually degrades the nicotine, causing it to lose strength, so I suppose it could go "off" too, if not stored properly (cold darkness -- mainly darkness, but cold probably helps a lot; some folks round here have viable nicotine that's been in "frozen" storage for 2-3 yrs). Pretty much anything organic can become "spoiled" at some point, but that's really the whole point of "processing," extracting active ingredients so that whatever carrier they come in doesn't become corrupted -- aspirin keeps a lot longer than willow bark! Refined sugar keeps FAR longer than sugar cane... Extracted nicotine would certainly have a far longer shelf life than the tobacco plants it came from, but that doesn't mean it's immortal.
Andria