Depends on time and task.

To steal from wikipedia here, an example of a Middle ages vanguard (literally “advance guard”) would have been the ubits tasked with first up duties. So..

Scouts

Engineers (to clear obstructions from the road)

And even some messengers/diplomats (to reach towns first and basically say “ok, the rest of the army is coming up, do you guys want to just surrender now? Or do we have to bother setting up the whole siege thing? Come on. Save us all a headache and just surrender eh? We’ll give you a nice deal)