You might have missed under "Uses for vigour" that you can spend vigour to increase a combat roll by the number of vigour you spend. e.g. if you fight a Wyrm and you roll a 5, you can spend 3 vigour to increase it to an 8, which does 1 damage to a Wyrm at full health. After that the number you need to roll goes down as the Wyrm's health lowers.
Does that make sense?
I'll update the rules to make it clear that you can increase the combat roll past 6.