Actually, Costco carries Biotene products, buually behind the pharmacy counter, and they are really cheap there. A bottle of the Biotene mouth spray is only about $2.75 there as compared to $5.99 at Walmart.
With that said, I do use the Biotene spray at night if I wake up and my mouth is very dry/sore, but generally, it is very temporary relief(an hour or two). Generally, I find regular water intake with small, frequent sips is the best bet. Also, nothing beats sour stuff like Sour Patch Kids or IceBreakers Sours. They will defiantly get the saliva flowing and if you happen to have a stone blocking a salivary gland, with regular use, they can actually help to dislodge it.
Also, if you use mouthwash, get a brand that is alcohol-free. Also, I've found that if I wait till right before bed to brush my teeth and don't vape afterwards, I don't get as much dryness or soreness in my mouth during the night.
Oh, and when you are Vaping, try not to have the tip of your young near the vapor as it enters your mouth. Not only can you conceivably burn it repeatedly with hot vapor, but the repeated direct exposure will kill taste buds, which in turn causes your mouth to feel even more dry and sore.