As I understand it, if the Active player commits to a story, there is an icon struggle for each applicable icon even if the defending player didn't commit any characters; ie I commit "Friend of the Family" with 4 Skulls to a story, my opponents doesn't oppose. There is still a Combat struggle, which I win, even if there is no opposition, so any events or effects triggered as a result of the Combat struggle still occur.

Thus I interpret "Vengeful Mob" such that if my opponent commits and I don't and s/he wins the Combat struggle I can trigger that event, even if I didn't commit to that story. Am I correct?

Syndicate
Vengeful Mob

Type : Event
Cost : 2
Subtype :
Game Text: Response: After you lose a C struggle, choose an opponent's character. If that character has fewer C icons than all of your characters that are not committed to a story, wound the target character.
Flavor text:
Illustrator: Max Bertolini
Collector's Info: The Antediluvian Dreams F47

That is correct. If you don't defend, they win the C struggle, you loose the C struggle and if they meet the criteria (have enough uncommited characters with more total C than your character it will have to take an extra wound.

Note that this wound is extra, not in place of the standard wound you would receive if you had committed.

This is critical! All those cards with "If you lose a struggle by X" or "If you win a struggle by Y" are applicable to undefended struggles, which means in some cases, where you get a powerful effect when you lose, it may be better to hang back. And, as Active player, this introduces another factor when calculating how many characters to pile into a struggle ... you may want to limit the margin of victory on a given Icon to avoid certain effects.

Thanks for this important clarification, now I have to go back over my cards and reassess how to use this effect. I didn't really understand its potential before.