Question from a non-native speaker: the Sensational Distraction shuji says, when making a Social (Fire) check targeting one or more characters:

"The target treats their vigilance as 1 lower..."

My question: Is that one opportunity per target per -1, or one opportunity pet -1 for all targets?

The text says "target treats", not "targets treat", but it also says "their" instead of "its"...

'Their' is fine when talking about a single person; it's a non-gender specific version of 'His' or 'Hers' without being specifically non-gendered (and non-human) like 'Its'.

You get a change of vigilance of 1 in each target per spent.

The number of targets is irrelevant because Sensational Distraction is not an action, it's triggered whilst making a check that already has defined targets; it affects every target of that check equally.