The FTB Forum is now read-only, and is here as an archive.
To participate in our community discussions, please join our Discord! https://ftb.team/discord
Open Launcher and "Apply Filter" for Minecraft 1.5.1 (or whatever version you want)
Select a pack that you don't play (I used "BronyModPackv2.8.0")
Launch the pack for the first time
Now close the game and re-open the launcher
Select the Mod pack you chose and click "Edit Mod pack"
Move all "Mods" to the disabled side
Move all "JarMods" to the disabled side
Move all "CoreMods" to the disabled side
Close the smaller window when you have all mods disabled
Launch the pack and you will see a Vanilla version of Minecraft, but it will still have the Logo of whatever modpack you used (no mods though)
I hope this helps. If it does, could you perhaps give my thread a glance? http://forum.feed-the-beast.com/threads/ftb-ultimate-lets-play-series.17900/
Also, don't forget which mod pack you "modded" or else you will have to check in the install directory to find out, which could get annoying if you use many modpacks! Your welcome!