So you do need Vivi then? I'm so fuck1ng confused now...how does talking to all the companions start Sera's romance?
You don't need to recruit Vivienne in order to romance Sera.
Talking to all companions isn't what starts Sera's romance. At one point in the romance, there's a gift quest where you have to talk to each recruited companion about it. If you didn't recruit Vivienne, it won't be required to speak to her, ofc. However, if you do have Vivienne recruited and you take her advice during this quest, it opens up an extra scene with Sera.
To actually start Sera's romance, you have to select that option from the dialog wheel under the 'Special' tree. I think you need to have reached a certain approval level first and completed her personal mission and prank mission before she'll agree to a relationship. This option will go away after "What Pride Had Wrought". Also, make sure you have exhausted all of her dialog options (they show up on the right side of the wheel). Doing her wartable missions helps as well.