The Teensy does not have a 5v pin, because it is a 3.3v processor. The Uno has a 5v pin because internally, it is a 5v processor, and it provides a 3.3v pin as a convenience.
Vin is the main input power pin of the Teensy. You are able to feed it 3.7 - 5.5v, and the Teensy will regulate the power down to 3.3v that the Teensy uses.
As a convenience, the power from the USB cable (Vusb) is connected to Vin. That way if you plug in a USB cable, it will power the Teensy without any additional connections. In general, the amount of power that you can get from a random USB connection is on the order of 100mA to 500mA (500mA is the most you can expect according to the standard, but some plugs now will give more than 500mA). If the total power requirement of both your Teensy and all of the sensors/leds/etc. is less than 100mA, then you don't have to do anything. If the total power draw is 500mA or less, you are fine, as long as your USB source provides at least 500mA (typically you would be ok, if the USB power comes from a wall wort, but if it is coming from a computer, you might only get 100mA).
Note, even if the USB plug is capable of supplying more than 500mA, you will not be able to get it, via the Teensy pins, since there are fuses that limit the maximum current to 500mA or so.
So, if you need to draw more than 500mA, such as powering 16 neopixels at full brightness, which needs 1A, then you need to use more than just the USB cable for power:
- You could provide 5v, 1A power to the Vin pin. In this case, you would want to cut the trace between Vusb and Vin, so that when you plug in the USB cable for programming, it does not power the Teensy. In theory, computers are supposed to protect themselves with a diode or equivalent, but you probably don't want to depend on that.
- You could provide 5v, 1A directly to your devices, cross connecting the grounds on the external power, and the grounds on the Teensy. In this case you would need to provide a USB cable to power the Teensy.
- If your devices need 3.3v but need 1A of power, you could cut the Vin/Vusb trace, and then feed the 3.3v from your external power source to the 3.3v/ground lines, as well as going to the devices.
If your devices are 5v you will need to make sure that the voltages to the device and to the Teensy are correct. Usually this involves voltage translation from the Teensy to the device, and it may need translation going from the device to the Teensy. Exactly what is involved depends on what the devices are. If you have a choice of devices, it is best to get devices that run 3.3v directly so that you do not need to do voltage translation. Since I have both 3.3v and 5v microprocessors, I try to chose only those devices that will work with both voltages.