Example:

Vigilance (unit effect): unit may not be countered

Unstoppable (Lore card effect): "before that unit is eliminated, it may counter."

I think vigilance trumps. Unstoppable assumes you would be eligible to counter (minus being eliminated), and you're never eligible to counter a unit with vigilance.

Vigilant wins.

Unstoppable Counters outside of the normal combat sequence; its Counter takes place before step 6 or 8, not 10 when Counters are usually performed. I think it is a special circumstance and that Vigilant doesn't come into it.

But it's still a counter. A vigilant unit cannot be countered.

Unstoppable Counters outside of the normal combat sequence; its Counter takes place before step 6 or 8, not 10 when Counters are usually performed. I think it is a special circumstance and that Vigilant doesn't come into it.

Cannot is an absolute term in FFGese.

The language is "may" and "may not" -- is it FFG's policy that "may not" trumps "may"? Where is this found? Otherwise, I don't see a clear answer to this question.

On the unit card the Riverwatch Riders clearly state, "Vigilant: Enemy units cannot counter this unit's attacks." Essentially additional effects that create counters, do not matter because even with a lore card because, it cannot counter the attack of the Riverwatch Riders.