I've been in the same situation several times and have found that too many people at those corporations have no clue what a vector is. I even had a reasonably high-up advertising rep at a pharma company send me a lo-res GIF and tell me it's all she had. Draw your own conclusions, but I've generally found that companies will seldom provide vector logos to outsiders, no matter how legitimate the use.

My solution has been to recreate the logo from scratch. In most cases, it's easier than you might think and it will also hone your Illustrator skills. Usually, the hardest part is identifying the fonts, but I've found this website to be very helpful:

<http://www.myfonts.com/WhatTheFont/>