Why was Victoria at the Prom Scene?

because twilight's director thought victori could be wacthing edward/bella while plotting her revenge

Since Alice can see the future, she could see that Victoria was not a threat at that time and did not want to ruin the Prom for Edward and Bella. If they were to be in any danger, Alice would have let them know. And who knows, maybe Alice did let Edward know by her thoughts and so he was aware of the situation but knew there was no danger yet.

They put her in there because she is watching bella and also they needed to leave off for New Moon.

Sometimes you have to put a hint in a movie not everyone has read the books, maybe it would motivate them to read New Moon. I do not think that there is anything wrong with putting a hint in a movie the director might have thought that. People who have not read the book wondered what had happened to her who knows, I had not even read twilight until I saw. The movie and seeing Victoria at the end of that movie made me what to read New Moon and the rest of the series.