Perhaps this is more of a question about the soldier recruitment process in the medieval ages. Because I can't understand the notion of a sane person willingly putting himself between two murderous masses of men.
Was there an incentive to being a vanguard, like higher payroll? Maybe it was all mercenaries? Or maybe all conscripts, given basic training and fed to the meat grinder?
The van is not an order of battle, it is the first of 3 parts in an advancing military formation (an army on the march) - the advanced guard's job is to seek out the enemy and pin it down for long enough so that the main body can swing in and crush them. A good clear example of this, abeit in the Napoleonic Era, was Marshal Jean Lanne's V Corp. Lanne was the vanguard commander for the entire Grand Armee - where he goes, Napoleon and the Guard follow. The maneuvers of the advanced guard are of strategic importance, allowing Napoleon to concentrate his forces and destroy his opponent piecemeal (or capture crossings and supply lines).
In the order of the march, being at the 'head' of a column was a place of honour. Not only do you not have to eat everyone's dust but you would USUALLY be on the right in the line of battle when it is drawn up - a spot reserved for veterans in the old classical line of battle.
Please specify a time and place. This is a topic which can be applied across all of (edit. Medieval) history and the world (probably). I think you could probably expand this question into one of bravery on the field in general. The vanguard is a fairly safe position if you're withdrawing and the rearguard becomes the most dangerous position, as an example. As you will no doubt see throughout the sub it is difficult to give a good answer when we don't know when or where we're meant to be talking about. Although with human emotion certain elements will likely be shared
The answer, drawing on my own studies, is socio-cultural rather than economic or legal. Fear, shame, honour, and bravery are all intrinsically linked in chivalric mentality. They are different sides of the same coin. By raising prowess and winning worship through arms as the pinnacle of socio-cultural adoration then you leave little room for cowardice. If you do not throw yourself into danger, vying for the chance to prove your fearlessness, then you may look like a coward compared to your brethren. Cowardice is consistently stigmatised and can eventually overwhelm the fear of injury and death. Thus reinforcing the cycle. Apparently it was quite an effective method as there many authors of chivalric texts emphasising the need for wisdom and prudence alongside prowess and bravery.
One could push the (economic) idea that those who fought at the front were more likely to gain spoils, and later ransoms, but I don't think it's true (for France and England after 1150 at least). Even the economic rewards for loyal and famous knights had socio-cultural roots. Laura Ashe's article ('William Marshal, Lancelot, and Arthur: Chivalry and Kingship', in Anglo-Norman Studies, v.30 (2008), pp.19-40) outlines how the expectations of romance far exceeded what could possibly exist in reality. It was still a powerful force, and Marshal's bravery (and skill) had much progressed his career from mesnie knight to Regent of England after 1216.
William Ian Miller (a scholar of the Icelandic sagas) has written many excellent works on fear, threat, and feud, well worth checking out. Craig Taylor's recent book Chivalry and the Ideals of Knighthood During the Hundred Years War, (Cambridge, 2013) also discusses these ideas.