The argument for taking Vengeance is largely based on the extremely unlikely scenario that Ulyssian can somehow convince five Immaculate shikari that he isn't really an Anathema
I Disagree with this. Vengeance is only about killing Seeds and there's a number of different reasons to do it that have been mentioned such as having someone as blatantly obvious as Seeds running around means both a massive hit to Ulyssian's reputation and great difficulty in being able to use his Necromancy for our advantage secretly.
No matter which route we take the Immaculates are almost certainly going to die and we'll either use the excuse that the necromancer did it or that they fell for an Anathema trick and Ulyssian had to kill them. We might be able to salvage the other two dragonblooded if we go Vengeance route but whether they live or die is mostly irrelevant, what actually matters is if we decide to kill them for convenience or whether to at least attempt to give them a chance to live.
I'm not against allying with other Solaroids but I just don't see us getting much use out of Seeds (he's just too blatant to deal with) and he's already proven himself to be a monster despite his good intentions. Besides if we want a nonaggresion pact we're in perfect position to get one, after all once he's dead he'll be in a poor position to fight.