There has been a default assumption about how Nick works that I think might not be true. If it is not, it opens up some interesting new approaches to dual wielding.
First, the Nick rule, plus the Vex rule for a point of contrast:
Nick said:
When you make the extra attack of the Light property, you can make it as part of the Attack action instead of as a Bonus Action. You can make this extra attack only once per turn.
Vex said:
If you hit a creature with this weapon and deal damage to the creature, you have Advantage on your next attack roll against that creature before the end of your next turn.
A default assumption on the use of Nick is that it has to be on the weapon that you make the extra attack with. That is, if you attacked with a shortsword + scimitar, the scimitar would be the offhand weapon because that's what you're making the extra attack with, and thus would have to be the weapon with the Nick property in order to gain its benefit.
However, if you look at the above property descriptions, you'll note that the Vex property specifies "with this weapon" in order to gain its effects. In fact, every single other weapon mastery does this — Cleave, Slow, Push, etc. The
only weapon mastery that does not include "with this weapon" in its language is Nick.
This doesn't really matter with the default dual wielding that the Light property grants because the only weapons that can be combined are all Light weapons, which means you likely want to use the other weapon's weapon mastery as your main attack anyway.
But it gets more interesting when combined with the Dual Wielder feat. As I described in my above post about attack ordering, you don't have to offhand your heavier weapon as long as you have Extra Attack available. However the heavier weapon will never have Nick, because Nick is a property that is only available on Light weapons.
However, because Nick is not restricted to "this weapon", that means that the bonus attack granted by Dual Wielder doesn't
have to be a Bonus Action attack.
Going back to the Rapier + Dagger combo, the dagger has the Nick property. It's now possible to consider the order of attacks like this:
Attack 1: Rapier (Vex)
Attack 2: Dagger (Nick); triggers Dual Wielder bonus attack
Attack 3: Rapier (Vex); made as part of the Attack action,
not a Bonus Action
In the case of a Rogue without Extra Attack, they can't use the rapier as the first attack because that's their only default attack, and they need to use a Light weapon to trigger the dual wielding attack. However, should they have the Dual Wielder feat, this benefit regarding Nick should still apply.
Attack 1: Dagger (Nick)
Attack 2: Rapier (Vex) as part of the Attack action
Which leaves the Bonus Action free for Cunning Action or whatever.