Your Vibrant came with Android OS 2.1
Froyo is Android OS 2.2
Basically, it adds the ability to download Adobe Air and Adobe Flash to your phone (lets you use a lot of Flash apps on websites, and watch Flash video, but not Hulu), the ability to make phone calls over WiFi (instead of the normal cell towers), some minor performance tweaks (as in so minor you may not notice), WiFi tethering via bluetooth (in theory) and one or two other things.
Overall, from 1.6 to 2.1 to 2.2, I've yet to see anything that made me go "OMG! I SIMPLY *HAVE* TO HAVE THIS VERSION!"
Only reason 1.5 to 1.6 was such a big deal (for me) was that it added Navigation to Google Maps
Pretty much, 90% of the people who are demanding 2.2 want it simply because it's newer, not because it actually will improve their life with the phone (of the other 10%, most will probably want WiFi calling because, if you live in an area with spotty T-Mo coverage, it's a total life saver)
As for the "how":
When T-Mobile releases the official 2.2 update, your phone should(!) automatically get the update and install it. I say should only because in the last 5+ years, regardless of phone, regardless of carrier, I have yet to EVER receive a single OTA update
If you don't get the OTA (because you have my luck), then there are several ways to get the update.
1) Kies. This is Samsung's update app. Assuming Samsung puts T-Mo's 2.2 update on (which they should), you'd be able to install Kies to you computer, follow the instructions, and have your phone updated.
2) ODIN. This is an app used to flash Android phones to new ROMs. All that's required is an ODIN "package" of the new Android OS ROM, and a few minutes to flash. No Root required
3) Recovery Flash. Using a tool from the Marketplace (Clockwork ROM Manager), you simply download the OS ROM, save it on the phone, and flash via the ROM Manager app. Requires Root.
Root means having Root Access to the phone. There are several methods for doing this (search is your friend!)