How to get to play-through 1 after starting vault hunter mode?

You need to go to Select Character from the main menu (you can't be in a game when you do this), then select the character you want to play Playthrough 1 with (even if they're already "selected"). When you click them it will give you the option for "True Vault Hunter Mode" (playthrough 2/2.5) or Playthrough 1 ("Normal Mode", AFAIK). Just pick the first playthrough from that menu and hit Continue, you'll find yourself back in Playthrough 1 exactly how it ended, all of your old missions intact and enemies at level 30 max.

This is different from how Borderlands 1 worked for this, but it takes away one extra menu every time you try to play the game so that's probably why it was moved.