If the VS was indoctrinated, they wouldn't have responded the way they did on Horizon. They would have leapt at the opportunity to join Shepard's team. I also don't see them ratting Shepard out to the organization they so openly despise either, I especially have difficulty believing that they (or indeed anyone else) would actually do a 'HAH I ARE EVILZ!' announcement during the reaper invasion.
What I'm saying is, if after all this bloody time of waiting to finally have the VS back in the game again, they drop a 'hah they were evil and indoctrinated from the very beginning. Guess you should have gone with Liara!' plot device on me. I will rage in a way that time and space could never be prepared for.
This is why I hate the idea of 'indoctrination' being used as a plot device in a story like this, because it means that we can have characters doing stuff that they would never do and of which makes no logical sense in any universe (like actually trying to kill someone on your side in the middle of full scale galactic war) and we don't even have to give them any kind of coherent motivation for doing it.
Might as well be saying 'the devil made me do it'. It's just stupid. If done right, in the case of Saren, it can actually make for interesting moments in the story as he's conflicted as to whether or not he is actually in control of his own actions.
But if all it does is amount to a 'now this character has become evil because screw you why should there need to be a motive', then all I see is a cynical attempt to not put effort into character motivations.
And frankly, even though I'm a paragon and I don't like Cerberus, I hope in game there's a better story justification for why they are suddenly eager to kill the person they just spent over two billion dollars and two years rebuilding, then just 'oh, they be indoctrinated y'all'.
Because right now it looks like they are joining Weyland Yutani, Armacam and the Umbrella corporation in the proud line up of 'dark and shady giant organizations that are stupidly evil for no good bloody reason'. Trying to destroy your greatest investment, and being heavily involved in trying to do that while in the middle of the bloody ARMAGEDDON pushes so far into the realm of 'stupid evil' that it's almost in need of its own classification.
Please for god's sake, I don't want any double cross plot twists at the best of times because they are cliche and stupid and run the risk of ruining characters that people in the Mass Effect community have come to know and love.
But if you must have them, for god's sake let it be a better justification then just 'they are indoctrinated' because a person turning evil for no other reason other then total commitment to mindless evilness doesn't make for good storytelling.
That said, if you must have characters be indoctrinated, you sure as hell better give people an option to save them somehow. Unless you want fanboy rage coming down on you hard,
As far as the 'who tipped off Cerberus' thing, I can't be sure who it was, though I honestly think it'll turn out to be a minor character or some random Salarian in Mordin's team, but I doubt strongly that it could be one of the squadmates.
One thing though, was it ever said that there would be a traitor? Because so far all I'm hearing is rumors. And I hope like mad that that's all.