I read about a highly-regarded One Click Lag Fix* app, but was nervous about trying it as it required rooting the phone. Plus, I had some problems getting the phone into recovery mode to root it. But today I was finally frustrated enough with the lag to try again, and successfully root the phone and apply the fix. I didn't brick it and it seems snappy so far, though a lot more testing will be required. I just wanted to do this quick write-up to address some issues that other frustrated Vibrant users may encounter, so they can hopefully Google this information for help like I did.
1. Download and run OCLF
I checked the Android Market to ensure that I had the latest version of One Click Lag Fix (OCLF). I then opened the app from Applications, and ran Root Device Recovery. The app confirmed that update.zip was installed, and I needed to reboot the phone into recovery mode to select it the file and root the phone.
2. Back up data
Before rebooting the phone to root it, I mounted it via USB on my Mac and copied over all data from both internal and external SD cards. I also verified that update.zip was in place in the root directory of the internal SD card.
3. Boot into recovery mode
Here's where I had a lot of trouble until I finally found correct instructions through Google. I had to simultaneously hold down both the volume up and down buttons and the power buttons until the Vibrant screen appeared, disappeared, then appeared again. Then I let go of the power button but kept holding down the volume buttons until the tiny yellow text of recovery mode appeared.
4. Root phone
Here's another place where I had a lot of trouble before finding the correct procedure. The OCLF instructions said that an option "apply sdcard:update.zip" would appear in the list, but it did not. Instead, I had to select (using the volume buttons as up-and-down keys) the option "reinstall packages". Only then did the phone install the update and reboot.
5. Install fix
After rebooting normally, I opened the OCLF app again, and granted it superuser permissions when requested. It took a bit of time for all of the available options to appear. Once they did I installed EXT2 Tools, then once that was done, installed the OneClickLagFix V2.2+, using the default amount of space suggested for apps (I believe it was 879MB for my phone). The app set the phone to flight mode, killed off other apps, and applied the fix. It only took a few minutes.
6. Test fix
After rebooting I launched some apps that had been notoriously laggy for me, including K-9 Mail, Gallery, and MyTracks. All seemed snappy. Again they will need several more days if not weeks of testing before I am satisfied. For now, I'm just happy I didn't brick the phone.
I hope this writeup was helpful to someone, though more likely a Vibrant user Googling for this info than any of my friends regularly reading this journal. The Vibrant is really a wonderful phone when it works correctly, and I'm hoping the Froyo 2.2 update will fix this lag once and for all when it finally comes out (looking like not till after New Year's at this point).
Update December 29: Phone still works fine; not incredibly snappy but not laggy either. Still waiting for Froyo (or, at this point, Gingerbread) for a permanent fix, hopefully.
* This app, though easy enough to use, is absolutely not "One Click".