The VIN pin on the Teensy is connected to the +5V coming from the USB connector. If you wish to power the VIN pin directly with your own external source you must do one of two things:
1) Never connect your own supply AND the USB at the same time.
OR
2) Cut a track on the Teensy PCB that permanently disconnects the USB from VIN. You can now connect your own supply at the same time as USB safely, but the USB can no longer be used to power the board by itself, Your extenrnal supply must be connected to make it work at all.
Okay, there's actually a 3rd option as well. The track you cut is between two solder pads. This is so that you can restore USB power by solding the pads together.
3) Cut the track between the pads, then wire in a SPDT switch with the pole soldered to the VIN pin, one throw soldered to the USB power poad, and the second throw soldered to your external source.
This allows you to switch between USB power and external power safely.