When they get the butter knives out
Ah, but voulgiers don't have butter knives, they slice les baguettes with the voulge only. I found that they do very well with Spearwall on, able to hold sword infantry while inflicting heavy losses.

Out of curiosity I ran a completely unscientific test.... a unit of Voulgiers with 2 armor upgrades against plain Dismounted Feudal Knights. Left Guard Mode and Spearwall on during the knights charge, then turned Guard Mode off. Voulgiers left with 57 out of 120, DFK left with 29 out of 120 - then the knights ran. Not sure what lesson to extract here but I suspect something about the importance of armour upgrades :)

Now if I could just get my hands on some Aventuriers in the campaign...