I have a 15 minutes piece that I would like to add to Vimeo. It was shot in 1080p on an EX1. I am trying to export it to a smaller quicktime so I can keep it under 500mb (it's 700mb exporting at the default settings).

What's the best method of keeping quality but reducing size?

Export using Quicktime. Select the Lan/Intranet preset for Exporting a Quicktime Movie.