Inspired by MVL, I played 6.a3 in the advanced French and won against a much higher rates opponent

I'm referring to the incredible game MVL won against Richard Rapport lately.

very well played!

What were you playing instead? A3 is main move as I know.

I usually play 6.Be2. I actually don't know the theory well so I often struggle to defend d4. I often have to give up on castling and play Kf1 after Bb4+ happens for instance.

6. a3 is the main line, Maxime Vachier-Lagrave did not invent it.

6...a5 is not main line, 6...c4 is.

I often play this line, but nobody ever played 6.a3...in case I all the same play 6. ...cxd4 and 7. ...Ne7
In this match, checkmate arrive for black kingside underdevelopment...

It may be theory, but 6.a3 is not an intuitive move at all. It's not a developing move and it seems to me that its sole purpose is to prevent Bb4+ later. So I'm not surprised few people play it. I, for one, never thought about playing it until I saw this MVL game.

6 a3 originates from Louis Paulsen. The idea is not only to prevent ...Bb4 or ...Nb4, but to threaten 7 b4. That is why black usually closes 6...c4.

After a3, c4 seems to make sense.