My understanding is that going to a vigil Mass does not fulfill a Sunday or a Holy Day of obligation. Do I have that right?

Recently I heard someone ask "where are you going for Mass on Christmas?", the woman replied, "Oh I already went". It was around 7pm on the 24th and somehow she thinks she had already fulfilled the obligation. Did VII allow the N.O. to fulfill the Sunday obligation by going to mass on Saturday evening? or where did this idea come from?

I'd like to advise someone against that practice but I don't know if there is anything specific that I can point someone to, like a encyclical, letter, cannon law. I used to go to the N.O. church before finding the traditional Mass but even then I don't think I ever "took advantage" of the vigil mass as replacement for the day of obligation.

I heard that it's a thing in the NO, even my SSPX priest said it doesn't count for Sunday. A NO family member I know went on Tuesday for Christmas... makes me sad.

In the Novus Ordo they have an "anticipation Mass" the afternoon/evening before a Holy Day or Sunday.  The "anticipation Mass" is not a Vigil Mass, which is actually a completely different Mass.  They say the Novus Ordo service for the actual Sunday/Holy Day during the afternoon/evening of the day before.

Vigil Masses do not fulfill a Sunday/Holy Day obligation.

Anticipation Masses DO fulfill the obligation for adherents to the Novus Ordo Religion, that is, if they haven't been dispensed of the obligation by their "bishop", which happens frequently.  For Christmas, Novus Ordo churches near me had Christmas services as early as 4:00 pm on Christmas Eve.  Even the FSSP gets in the act on this.  I didn't look this year, but the FSSP has had "Midnight Mass" as early as 8:00 pm on Christmas Eve in Indianapolis in the past.

Of course, all of this is for the Conciliar sect, not the Catholic Church.