The more I think about new career options for Sienna, the less I’m convinced that the developers are as constrained by her current careers as people think they are.
The Warhammer Fantasy Roleplay Career Compendium (2009) explained the game’s career system thusly:
As a character advances over the course of a WFRP campaign, the careers he adopts become a sort of journal of his life. The story behind the Rat Catcher who seizes an opportunity to strike it rich by becoming a Thief could be wholly different from the Rat Catcher who takes up arms and chooses to become a Shieldbreaker. Or the Apprentice Wizard who eschews the other wizardly careers to instead focus on acadaemia and becomes a Scholar. The countless other career combinations also have their own unique stories to tell.
Sienna’s next career doesn’t even have to be any kind of magic-user. A character doesn’t completely abandon or suddenly forget their old abilities when they choose a new path in life. Sienna would simply continue to make use of Aqshy in addition to the tools and tricks of her new trade.
You can argue that choosing a non-caster career and continuing to use magic would make her a hedge wizard and therefore a target for the Witch Hunters and the Colleges of Magic, but that already applies to her Pyromancer and Unchained careers. Only Battle Wizard represents a Sienna who has completed her training and is thus an official member of the Bright Order.
That said, if the recent Lohner’s Chronicle entry really is setting up Sofia Fuegonasus as a sort of dark mirror for Sienna that she must confront again, one of the most fitting options for Sienna’s premium career is the Master Vigilant mentioned in the OP. A career for stealth, espionage, deception, and other dirty tricks up her sleeve. Since master vigilants rarely operate openly, the writers could even try to pull off the twist that Sienna has secretly been a master vigilant all this time. Saltzpyre’s reaction alone would be worth it.