Lauren was the recurring character of season 3 and her storyline, and the season, concluded with her death at Vaughn's hands, which means the storyline lasted an acceptable amount of time and didn't become boring.
For those who didn't watch Alias, Vaughn/Lauren mirrored Jack/Irina (Sydney's parents) completely. He's an agent who marries a woman who is in fact an undercover agent for an enemy agency (Irina worked for the KGB, Lauren for a fictitious organisation called The Covenant) and he ends up feeling pretty humiliated when her manipulation is finally discovered. I don't know why they always make the woman the traitor on tv - it's always more shocking that a woman would be so heartless to actually marry a man she doesn't love just for espionage.
Anyway - Sydney comes back from a two-year memory loss; as far as she remembers she was with Vaughn, instead finds him married to someone else because she was believed to be dead. I think when it's revealed midseason that Lauren is in fact a villain, everyone was relieved that they now had a perfectly good reason to hate her. In the season finale, Sydney is at a building site in Palermo and has a face/off with Lauren, until Vaughn appears to save the day and shoot Lauren dead. We all wish there was a Vaughn ready to shoot a few bullets into whatever bully or nasty person is bothering us!
A nice synopsis of the season 3 love triangle:
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Day 01 - What is your current favourite ship?
Chuck/Sarah Day 02 - What was your very first ship?
Tommy/Kimberly Day 03 - A pairing that needs to happen now?
Grissom/Sara Day 04 - The pairing with the most chemistry?
Xena/Gabrielle Day 05 - The pairing with the least chemistry?
Shannon/Sayid Day 06 - The best kiss?
Sydney/Vaughn Day 07 - The most heartbreaking scene?
Naomi/Emily Day 08 - The pairing with the most baggage?
Doug/Carol Day 09 - The most believable relationship?
Chandler/MonicaDay 10 - Why aren't these two married in real life?
Zach Levi/Yvonne StrahovskiDay 11 - What is your dream pairing?
Liz Lemon/Jenna MaroneyDay 12 - Who had the best wedding?
Phoebe/Mike Day 13 - What is your favorite television/movie pairing?
Christian/Satine Day 14 - What is your favorite book pairing?
Dar/Kerry Day 15 - What is your favorite real life pairing?
Mark/Ery Day 16 - What is the absolute worst pairing?
Will/Emma Day 17 - A pairing you thought would never work out, but did?
Sawyer/JulietDay 18 - What is the cutest pairing?
Chuck/Sarah Day 19 - A pairing you've rooted for since the beginning?
Grissom/Sara Day 20 - The "can't stand the sexual tension anymore" pairing?
Mulder&ScullyDay 21 - A pairing you like and no one else understands why?
Rachel/Joey Day 22 - A pairing you hate and no one else understands why?
Barney/RobinDay 23 - A crazy love triangle/quadrilateral that worked out great?
Day 24 - A crazy love triangle/quadrilateral that worked out badly?
Day 25 - A pairing that was/would-be adorable, but could never work out?
Day 26 - A pairing that you hated and ended up loving?
Day 27 - A pairing that you loved and ended up hating?
Day 28 - A pairing that you will never understand?
Day 29 - What ship had the best proposal?
Day 30 - Your favorite ship forever and ever and ever?