Unless you're Italian and are conducting an orchestra, it's best to avoid saying the word "vivace," if you can. Otherwise, you might overpronounce it with an exaggerated accent. I still remember one conductor doing this; even though it was over 30 years ago, I still remember his over-the-top pronunciation as if he'd said it earlier today. Sometimes, though, that term is in the score, and there's no getting around saying it.
Even then, though, it's probably best to just tell the orchestra, "Remember, this movement is lively and fast."
