Vivienne is a master player of Orlais' Great Game. She is ALWAYS wearing a mask, and NEVER revealing her true self, thoughts, and emotions. But I think she genuinely does let it slip for just a moment when she gets last words with Bastien. That is the face of a woman who has just watched the man she loved die, despite all she'd done for him. The fact that, by the time that his sister and son arrive at Skyhold, she's replaced the mask and is taking advantage of his death as best she can is completely beside the point.
Because she had pinned her star to Bastien, everything that she did afterwards was in an effort to ensure that she could keep going after his death, because she NEEDED that protection - the Circles are gone, the templars may not be what they were, but they're still a threat to any mage, and there's all that she had internalized as a result of having to work within that system to get where she was. She had lost her lover, who was effectively also a patron for her, and, given that Morrigan was in the Orlesian Court, her position as the Imperial Enchanter is probably in doubt, along with the fact that, without the Circle, her taking the position of Grand Enchanter or First Enchanter of a Circle means nothing, she had to do what she needed to in order to protect herself. So yes, she doesn't take the time to mourn, just to turn it to her advantage, because that's the way that she's always done things, in order to ensure her own survival.