The Easter Vigil Mass absolutely must begin after sunset. Phemie just quoted the law on this. When Rome uses the term “Reprehensible are those abuses” it should be clear that such is not to be done.
Before (about) 1970 it was common for the Easter Vigil Mass to begin in the morning of Holy Saturday, because by canon law, all Masses had to begin before Noon. That caused a serious disconnect, to say the least. It just made no sense celebrating an event that happened in the middle of the night, but doing so in the morning hours. Thankfully that was corrected.
In some places (and I know it happened, and still does in the US) some parishes unfortunately begin the Easter Vigil Mass too early. Every year since I became a priest, around Ash Wednesday, we get a circular letter from the bishop telling us in no uncertain terms that the Easter Vigil must begin after sunset. The fact that we keep getting it is a good indication that some pastors still “don’t get it.” It’s frustrating.
Anyway, we have a habit of calling the Saturday evening Mass a “Vigil Mass.” That’s not always correct. A vigil, by definition, must be done during the night. That is what the word means, after all. There’s nothing wrong with a 5 PM Saturday Mass as the Mass of Sunday; in fact, it’s one of my pet peeves to post in support it. Yet, if it’s before sunset, it doesn’t meet the definition of a “vigil Mass,” although it does fulfill the Sunday obligation. Anyway, that’s just the definition of the word. It’s not a topic I choose to make an issue; just clarifying the definition, since that’s the topic here.