Hello everyone! I have just found out how to fix the problem where a custom modpack would open in Vanilla Minecraft instead of Forge. I don't know if there are other tutorials on this, but I haven't seen one and have seen this problem a lot with no answer. Here I go.
First, this is if your modpack works and can be opened up by the launcher.
When the launcher installs your modpack, it does not include the modpack.jar with your pack. Here is how to fix this. Just go to /%appdata% (Or wherever Technic is installed)/.techic/(Your modpack name)/bin
If you look in that bin you see the modpack.jar isn't there. If it IS there, this isn't your fix. Just put your modpack.jar as you have it in your pack in the bin folder.
It should now work and be loaded up in Forge.
But there is another problem. The mods aren't loading up either. There is a folder in your modpack folder if you leave the bin that says your modpack name. Open it up, and all your mods and stuff should be in there. Just do what you did there in
/%appdata%/.technic/(Your modpack name)
and it should work.
I'm sorry if someone else posted this, or this bug only happened to me, but if it helped you please tell me below. Thank you!