All I can say is my theory seems to be correct (for a part at least). Vasari rebels and TEC join forces.
What worries me is their cause. In various sources I've read so the vasari can take the tec with them on their escape. (which is also suggested by the end of the cinematic). But I'd prefer the idea (and I recall a dev saying this in an interview) it is to join forces and face the enemy head on. It makes sense, the TEC stood no chance against the vasari when they invaded. But a few decades later they can fight them head on. The vasari might think they stand a chance if they and TEC and perhaps the Advent rebels team up.
Speaking of which, I've been working on logging factional relations, and I'll probably get around to making a thread about them all once all 3 races are added.
One thing I noticed which lends more credence to this theory is that during my short game as Advent Rebels vs. all 3 other factions, my base standings with Tec Loyalists were remarkably good at only -1.35.
(Standings with the TEC Rebels were, predictably, an enormous -11.25.)
Edit - Also "perhaps the Advent rebels." Just going to point out that the fleet at the end of the cinematic had all 3 races' ships in it, the Advent ones were just kind of obscured by the Titan.