Rules FAQ:
- 
Attacker has Desperate Attack revealed, defender plays Vengeance for Elia == claim 4
 - 
Defender has Desperate Attack revealed, plays Vengeance for Elia == attacker's normal claim. (Source)
 
War.
While you are the defending player, raise the claim value on the attacking player's revealed plot card by 1.
Link: Decklists
                                                    Rules FAQ:
Attacker has Desperate Attack revealed, defender plays Vengeance for Elia == claim 4
Defender has Desperate Attack revealed, plays Vengeance for Elia == attacker's normal claim. (Source)