The problem with
most “truffle oil” is that it’s truly vile, and if you put it next to actual truffle infused oil no one would confuse the two. The problem is that so few people have actually had truffles, never mind truffle infused oil to know. To a greater or lesser extent the same is true for vanilla, although far more people have had the real deal as a reference point.
Still vanillin is artificial, but hardly a terrible thing in its own right. As you said there are recipes where the flatter note of vanillin works better than the more subtle and complex floral notes of true vanilla. I’d argue however that truffle oil is always disgusting, and the issue of it being somehow fake or fraudulent is secondary to how nasty it is.
Still, in an application like ice cream real vanilla in generous amounts is so much nicer than artificial. Of course ice cream producers figured out that most people aren’t able to tell the difference, lacking experience with a reference point. They realized that black flecks in the mix were what people really looked for, so a market emerged. After vanilla extract is made the bean is totally devoid of flavor, but it’s now ground into flecks and sold as an additive and can be legally listed as “vanilla bean” in the ingredients.
On a semi-related note a friend recently told me that “most cinnamon is fake!” It turns out he learned that a lot of cinnamon used is cassia cinnamon, not Ceylon cinnamon. I had to explain to him that they’re both cinnamon, and both great in different applications. If you want bright, spicy cinnamon then cassia is the way to go. If you want floral cinnamon then cinnamomum verum is the way, and if you want something in between with a natural sweetness, Vietnamese cinnamon is it.