It's been over 6 months since Froyo was released, but for Vibrant owners, this has been a very frustrating time.
If you want Froyo for your vibrant, you can download the update software, but it will require Windows.
In order to get the update to work on your phone, or if you are having any issues, go into Settings->Applications->USB Settings and select "Samsung Kies". You will then need to return to the home screen, and turn off the phone before plugging your phone into the computer via USB. As an additional note, you will need to disable any custom home screens, and return to Samsung's launcher.
The update has several problems, such as the fact that the download from the internet and the download to your phone are a single transaction. This means that if you stop in the middle of your Vibrant Froyo update, your phone will have to re-download the entire update via Kies Mini. It's still ridiculous that there is no OTA update, and that you HAVE TO HAVE A WINDOWS COMPUTER to update your phone, but this is the ridiculous proprietary world we live in. We will have to deal with it at least until CyanogenMod is available for the Samsung Galaxy S vibrant.
Another important note is that this update will remove any rooting you have done. Rooting will in theory be possible again, but I have not yet found information or instructions on that.