I was able to start the Vesunna quest with an older save and the save patch. However, it is still quite bugged. The image for Vesunna (the character) is apparently not present or it's not looking for the right one. To start the quest, when asked to select my party I had to look for the "required character" icon (the cat bell) with no image or name. That's Vesunna. She was at the very bottom of the list. I had to put her into my party so it's just the MC and the unnamed, imageless guest character (Vesunna).
Unfortunately, the game crashes if I open the menu with her in my party. So... no saving till it's over. Fun.
The same thing is happening for me with Twilia when doing the Rulwe companion quest as well. And I'm going to assume any quest with a guest character.