thank you, OP, for making me waste my day reading lore at destinypedia
now that I know a bit more than I did before, I think the whole "vex can resist taking" is a local anti-lore bullshit made to give more background to that side missions chain
also, since vex worship Darkness and are kind of a decease for our universe (and maybe others too), there's no point in alliance
well, looking at the scale of events happening in that universe, I have no clue how puny animated dead husks that are guardians, are capable of successfully fighting such massive supernatural forces. even the Traveller itself seems to be a pretty weak entity compared to Darkness and even its followers, been beaten by them twice already, and that's only what we are aware of
so, uhm, yeah, the whole gameplay of both games is kinda anti-lore bullshit...
upd: also, we have several unfinished storylines we already know won't be finished in DLC3 and likely can't all be finished in DLC4 (for example, black triangles in the skies and Savathun)
English
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<The Vex have not worshiped the Darkness since the Heart of the Black Garden was destroyed. The black triangles are the Darkness. Savathun is DLC4.>