Essentially, yes. There are variations on this based on the time period (and thus technology) as well as the theories of any given army. But broadly speaking, a vanguard could be any given army unit that is simply assigned that task. The next time they advance, it could be a different unit serving as the vanguard. Some armies might prefer to make the same unit the vanguard on a regular basis and train/equip it differently because of that. In some instances, it might be sending horse cavalry out ahead of the infantry, or it could be a specialized tank unit in WWII. I tend to think in the context of the American Civil War, so I typically picture infantry units ("skirmishers"). Usually they want to clear out any enemy units separated from the main enemy force (stragglers, scouts, or the enemy vanguard). Depending on who intends to go on the offensive, the vanguards might engage each other in a minor "battle" but they are never intended to carry out a proper fight, only to "feel out" the enemy, get them to retreat to avoid the main force, prevent the enemy from gathering intelligence about the size or position of the main force, or like I said before to coax the enemy into an attack on an entrenched main force.