We've got a little conundrum, and I've mentioned this in other threads, but out of the three female companions, all three of them fit "the mold", as it were, for a Bioware LI, where as aside from DHMG, none of our male companions fit that mold, though most all have developed their own followings, regardless.
So for a while I think the general assumption was that all three females were going to be LI's, but with the confirmation of Cullen as an LI, that becomes less likely since people assume there'll be an equal number of of LI's per gender and that if there are npc li's, each gender will get an equal number of those.
I've seen people doubting that we'll get six LI's. I think eight is a reach that definitely isn't going to happen, but that's just me.
So a few thoughts based on your post:
* I'm not so sure that I see Vivienne as fitting within the LI mold. Yes, she's absolutely gorgeous, but outside of that she seems to me to be more of the "unattainable companion" mold, like Varric and Aveline (and Samara from ME). Her haughty and superior attitude could very well be written as a "no lowly inquisitor is good enough for me" type of gal. It's just my gut at this point.
* I totally agree that I don't see 8 LIs in our future. That is so far above what we've had in DA 2 that the comment about resources being similar between the games would not make sense. Although, given that it's looking more likely that both Cole and Dorian are in after being told that at least one of them wasn't, I guess anything's possible.
* I am also not really going to rest my hat on the perfectly symmetrical distributions that people are putting forward. 1.) Just because we have 1 NPC romance (especially given that he's Cullen, who has a very strong fanbase), doesn't mean that we will have more than one; and 2.) If we do have more than one, I could very easily see them having different sexualities from each other. Just because something "makes sense" doesn't mean it's going to happen. Look at, well, pretty much every other game where it wasn't evenly distributed.
So, I'm sticking with Cass as straight, Sera as bi, and Scribe Girl as lesbian. It's totally based on my jaded interpretation of past tropes in Bioware romances and I'm perfectly aware that I might be completely wrong. In fact, I'm looking forward to being proved wrong because what I expect isn't what I want.