Rashi in the beginning of the sedra states that the extra vav in the beginning of the parsha is coming to teach us that just as the previous parsha was said at Sinai so too this parsha was said at Sinai.
This is very difficult to understand what the practical lesson is as common Jewish thought is that everything in the Torah was given to Moshe at Sinai, if so rashi’s question remains and the vav is extra.
I wanted to suggest that rashi believed that not all the Torah was from Sinai and hence why in a few places he states that the parsha comes from Sinai. However the difficulty with this approach would be the Mishnah in Sanhedrin which states that if someone believes that one letter is not from Sinai he is a heretic and unless otherwise proven rashi would go along with this Mishnah.
There are a few attempts to answer this rashi however I have not yet seen one which makes the answer better than the questions and in my opinion as long as the question makes more sense then the answer the question remains.