From what I have gathered, reason vests are popular in SF is due to daily weather fluctuations since they are light weight. I did not know they were popular in other cities/metros too. Especially in finance sector.
[EDIT] that is, The Official Preppy Handbook. The sequel's called True Prep and is, again, awful, do not read it, it has none of the fun, well-voiced humor of the first, it's bad, bad, bad.
I can't wear sweatshirts at my current job, but used to at my 3 previous jobs. Now, it's a button down shirt and either a sweater or vest. Ive plenty of both, but none corporate branded.
Vests are nice in that they can easily be removed and put back on.
Most of my sweaters are partial zip. I can't easily remove one without my shirt and undershirt coming with it (I'm a guy, but I still think its rude to go flashing my bare chest & back in an office).