I've finished the main story twice now and I haven't unlocked TVHM yet. I was told it comes after you report to Jack near the end but that hasn't happened. What's wrong?
Now regardless of game mode (multi or singleplayer) you will unlock the True Vault Hunter Mode after you've handed in the final story quest to Jack. Maybe you've just missed the popup window that informed you about it but it should be there.
Now remember you won't just be in the TVHM just like that. To activate it you will have to go to the Character selection screen and chose a character. Then you will be asked in what gamemode you want to play the game. It's important that you should try to get to level 28 first before you start with TVHM. While it's certainly possible to be successful at 26 or 27, the enemies are really a lot stronger than in normal mode so keep that in mind while playing.
Have you finished defeating the final boss? You have to beat the final boss and then you should get a popup that says that you have unlocked the "True Vault Hunter Mode".
It's unlikely based on your description of the situation, but it's also possibly you may have missed the TVHM mode restart. Try going to the character selection screen and looking for an option that should ask you if you would like to restart to TVHM mode or continue with the regular gameplay.
the game will prompt you to go ahead and restart in TVHM after killing the final boss. If you didn't do it at that time for whatever reason, you can select the mode in the character selection screen(a option will pop and ask you if you want TVHM or to continue in regular mode). You can go back to regular or harder modes(depending on unlocked modes) via the character select if you wish to farm legendary items for lower level toon's (and or help your friends within limits).