Important to note: at least in less movile eras, the vanguard could presictably be expected to encounter the enemy FIRST.

Even on a single fixed battlefield, thus their unit position in a battle formation, front of the group, right end of the group.

This was considered a very prestigious position for that reason.

In a fixed battle, the vanguard position would logically go to the “best” of your line units to strike the first blows.

But who that best unit would be could change. It could be decided by the leader of battle for that battle. There are plenty of examples of Viking or Celtic clans agreeing to fight together, but bitterly arguing over whose troops would have “the honor” of leading the formation. (To the point that there were even fights over the right to lead the following days actual fight)

On the other hand, some leaders might make a specific unit their vanguard unit, and continually maintain that unit as a vanguard.

To describe that in modern military terms, you could have 1st infantry, 2nd infantry, 3rd infantry, and you COULD select the best one of them at any given time to be your vanguard,

OR

You could say,

“We have an Army Ranger regiment. They’re going to take vanguard, because that’s what they’re for. We specifically select, train, and equip them that unit to be our vanguard unit. Thats why the get extra pay and special uniform markings and the prestige of being on the first-string-all-star team”