I presume you were thinking of 'vigintennial', and I very much hope that your teacher didn't tell you that this was useful word. In fact, it is so rare that it doesn't even appear in most dictionaries. This word is never used - forget about it.

20th - I'm from USA, I've NEVER heard anyone use the other term

"Vigintennial" has a French/Latin origin, and is part of (very) formal English. Almost no-one uses this word, so we use "twentieth anniversary" 99.99% of the time.

I think (20th anniversary) is more famous good luck

I can't imagine why anyone would tell you to use or even learn the word 'vigintenniel.'