I shot SD 720x480 on Sony HVR-A1 with widescreen (16:9) and edited on FCP.

Sunmi + Peter Wedding Highlight on Vimeo (http://vimeo.com/5904646)

Had hard time exporting it cuz recommended setting on Vimeo (Compression guidelines on Vimeo (http://vimeo.com/help/compression)) still gave me squeezed picture.

Got it exported ok with "current" ratio, instead of custom 640x480. And it looked perfect on my computer. But when I uploaded, aspect ratio changed to 4:3.

I saw it said "853x480 for widescreen DV" but didn't try it because the original clip was smaller (720 x 480). Was that what I had to do with mine? What do I have to do?

I use the Cram compressor plug-in Vimeo presets and they really work for me.

You might check that plug-in out. It is very cheap.

It sounds like it is a pixel aspect ratio problem.

The first attempt you did sounds like you export your video with a 1.33 pixel aspect ratio and then when uploaded, Vimeo read them as square pixels.

Have you tried exporting at the frame size Vimeo recommends with square pixels? That might work.

All the best.
Chris.

DV is rectangular pixels. You need to convert them to square pixels for viewing on the internet. If you look in the Compressor Inspector window you'll find a number of places where you can make your videos square pixels.