I hear this charge from time to time, but I've rarely heard much reason to believe in it. The argument that Vivienne will inevitably jump ship as the tide changes is big on inevitability because, hey, politician and the Game, but very little on background or history.
There are a significant number of points in the series in which Vivienne's politics aren't turned by the tides of the moment, and her loyalty/favor not tied to the patron of the hour, and neither a fair-weather ally nor an ideologue of least resistance.
Vivienne's views are consistent across the game and our knowledge of her, past and present. She expresses and holds them before the rebellion (Cole's flashback insights), she holds them in the face of the rebellion when her power base of 'representative of the mages' is currently in revolt, she holds them when she is a member of the Inquisition, and she holds them if she gains real political power. She doesn't mollify or change them to gain favor- no matter how much the Inquisitor agrees, or disagrees, or actively opposes, Vivienne's expressed and practiced views are consistent whether she's in power or not, whether they're popular or not.
This is not what one would expect from a semi-sycophantic 'curry views to the current powers' suggestion, or the 'she'll change it soon enough.' Unlike, say, bloody mood swing Leliana, who's undergone more than a few ideological conversions over her career. Or ever ever-amiable no-drama companions like Garrus from Mass Effect, who's as likable (and likes the player) no matter what the PC does.
Nor does Vivienne have any exceptional history of jumping ship for higher tides- if anything, she has a history of jumping for the underdogs and making them into positions of power. Most notably- Court Enchanter, once a magical fool, turned into a position of relevance. Her dalliance with the Duke, while eventually an independent power base, was initially a drag on both of them if WOT 2 is what I've heard. She reaches out to the Inquisition when it is still a minow in Thedasian politics, unlike the still-standing Chantry which is ripe for political feuding and power grabs. When faced with a practically unkillable hyper-magister who, the future tells us, can fight and beat Thedas, she sticks by even though there's no clear way to beat him.
Vivienne clearly isn't risk-adverse: she takes a number of gambles, and efforts, that pay off in the long-run but not because they obviously would. She'll take risks, and make the most of them, and appreciates the power from them.
But what of the things she doesn't do? Or rather, where are her betrayals for the advantage, present or prospective? In the face of a Mage Revolution that offers the 'chaos is a ladder' sort of careeer advancement, Vivienne doesn't ditch the collapsing Circle system for a chance to play Revolutionary Leader (a role her political acuemen, and common sense, would put her well better than Fiona). Nor does she play Collaborator with the Templars, which one would suspect if she simply trying not to be dragged down. In the face of a 'join us or die' rebel wave, she stands fast- in the face of a pending Templar victory as the war sees the mages driven to Redcliffe and nowhere else, she isn't working with or for the Templars to be on the winning side.
Vivienne is as stalwart an ally for the Inquisition as any other we find. There's not even a hint of her being involved or even interested in schemes to break with its greater goals: no one is seeking her out to play a part in the Orlais plotline as her ticket back to the Court Enchanter position, she is not seeking out any Venatori in hopes of betraying the Inquisitor and setting herself up as a Priest-Queen of her own corner of Thedas, she doesn't express any interest in Corypheus despite him being established as having the means, intent, and even future-foreknowledge success of actually conquering Thedas.
If there's any 'Lord powerful enough' to make the tides change and encouraging a jumping of ship, it's Corypheus. But she doesn't.
And, to my knowledge, never has. So why the exceptional suspicion?