BrookerT wrote...
funny thing is, Vigil was a deus ex machina, and everyone loves him
Incorrect. Vigil gave us an information dump. The plot device that made our victory possible was what the Protheans did 50,000 years ago (reprogramming the Keepers, leaving beacons which we've enteracted with twice, and a program to greet anyone who comes for the Conduit). And we know about Protheans and the Conduit before we even gain Spectre status.
The Cruicible is also not a deus ex machina because we know about it early in the story (in ME3's story not in the series story) and that it's a possible weapon powerful enough to stop the Reapers. Which it is if you go with the Destroy ending.
Once more with a feeling...
A deus ex machina (
/ˈdeɪ.əs ɛks ˈmɑːkiːnə/ or /ˈdiːəs ɛks ˈmækɨnə/ DAY-əs eks MAH-kee-nə;[1] Latin: "god from the machine"; plural: dei ex machina) is a plot device
whereby a seemingly unsolvable problem is suddenly and abruptly solved
with the contrived and unexpected intervention of some new event,
character, ability, or object.
Nothing we know about the Catalyst prior to the rising at the end even hints that the Catalyst is an AI. The name even implies it's merely a firing device that makes the Cruible work. Hell, the mission begins with the player and all involved parties believing the Citadel is the Catalyst. And then the Citadel shows a previously unknown intelligence and offering untold powers (we know it kills Reapers, you could argue that TIM's interest proves it can control them, but no one can claim that synthesis was foreshadowed).
An example of a deus ex machina is Matrix Reloaded where Neo gains the unexplained and impossible power to kill machines in the real world.
Another example is the rail gun from the second Transformers movie which is never mentioned until it's called to defeat the giant wrecking ball bearing robot. Then it's instantly forgotten and never used again against any of the other giant robots waging war across Eypgt.