Does this have an expiration/use by date? Or does it just stay usable forever?

I believe it is forever, no expire date, these chemicals are so light, its more of an oily base than anything, so I think you'll be good for a while.

is this Propylene Glycol 400?

I don't know it does not say on the bottle. I purchased it for a Christmas train to make it smoke. It apparently has many uses and is used for e -cigarettes and many other uses.

I'm trying to make e-liquid for the first time. Could I just add vanilla extract and nicotine solution and let it steep? Suggestions appreciated!

I wouldn't use vanilla extract but you can try it. Go to http://www.todmuller (dot) com/ejuice/ejuice.php so you can safely figure out how much you need to dilute your juice. I would also look up perfumers apprentice to get good flavorings for the best price. I also use Nude Nicotine to get my liquid nicotine. And yes, you can use this propylene glycol

Propylene Glycol - PG - 1 Pint (18 oz net wt) - USP - Kosher - Food Grade - Safety Sealed resealable HDPE Container, Microwave (Remove foil Seal) Safe, Freezer Safe

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