It may not have been a true vision, but Trelawney was as firm a believer in what various divination tools told her that she thought whatever Lavender had seen was important. She listened intently, frowning when Lavender mentioned the two cards, neither of them signifying anything good.
Lavender wasn't really making much sense, but the important thing was that dreams had a power of their own. And if Lavender was dreaming the same things repeated, it had to meant something. it may just not have been entirely literal.
But then she hit her with that last thing - that the woman had actually been there, and Trelawney actually started a bit. "Ah," she began, the sound as though she understood. "Perhaps you are seeing something at that. Perhaps it is that you are having these glimpses that are opening you, bringing you closer to the possibility of danger. Something the knowledge of danger is what brings that danger to you. Finish your tea dear, and careful, you'll burn your tongue. Still quite hot. I must read your leaves."
Whether mentioning Lavender burning herself was a prediction or simply a causion wasn't too clear.