The marketing around Vivienne was very poorly done if you ask me, I hope Bioware learns from it.
*First off, She was one of the first companions revealed as such we got more time to get familiar with her implying she would have an important role, and in Bioware terms that means LI (see all screenshots, trailers, she always figures prominently)
*Secondly, people were very early stating Vivienne was a great option (so far) when it concerned potential LI, moreover she always seemed to be a logic choice for LI. Bioware never did anything to make us think different, moreover they waited till the very end to say she wasn't. They should have done more to put don the rumours rather then let it build up to something I can only call an anti-climax. Eg. the announced Iron Bull very early as BI, which was I fun surprise, but damn, they could have announced then that Viv was no LI instead and wait with Iron Bull till this week, it would have made a nice surprise for many to have IB announced and it would have at the same time have avoided for many to be very disappointed with the late announcement of the Non-LI role of Viv.
*Thirdly, this just sucks, big time, damn Bioware, bad communciation, really