VDD on crystal oscillator to GND or +3.3V? (ESP8266EX)
There are two different products that come in similar packages. You can get either a bare crystal &mdash; which is what the schematic is showing &mdash; or you can get a complete, self-contained oscillator module (the one with the Vdd pin).

Both products are available in tiny SMT packages, which have four pads, one on each corner. For the crystal, only two of the pads are required, so the other two pads are typically connected to ground, for shielding. The oscillator requires three pads &mdash; power (Vdd), ground, and the output signal. The fourth pad is sometimes used as an output enable or frequency control voltage connection, but otherwise it is either not connected or connected to ground.