Someone gave me a bottle of pure vanilla extract. It must be 20 years old (the top is sort of fused to the bottle, so it's very tight). Is it safe to use it?
I would think it's fine (maybe even better than 'fine'), provided that it was properly stored, and wasn't exposed to light or excessive temperatures. I could be wrong, but I don't think you'll get sick from using it (if it has been improperly stored and gone off), but rather it would just have a diminished taste. If it was unopened these many years, I think it's a better bet that you've been acquired a nice gift. Provided you can get the lid off.
Sure, pure vanilla extract is 35% alcohol. You can keep vanilla extract indefinitely, it ages like fine red wines.
"Keep it in a cool, dry, relatively dark place; like any other flavor product, the flavor will degrade if exposed to extreme or variant temperatures, moisture or direct sunlight." quoted from Vanilla Flower FAQ's. So, if it's been stored improperly, you'll not get much culinary use from it, but it won't hurt 'cha!
Enjoy your well aged vanilla.
"Safe"? Probably. It's mostly alcohol so it's unlikely that it will be unsafe to use. "Good" is another thing. But that will be obvious when you open it.