Before you use the VR headset for the first time, you should measure your inter-ocular distance (IOD, the distance between the centers of your eyeballs, also often referred to as “inter-pupillary distance (IPD),” although that is technically incorrect). This is an important parameter for VR rendering, and in practice it determines the scale at which you see the virtual environment. If your real IOD is off from the IOD configured in the VR software by x%, you will see the virtual world x% larger or smaller than intended, which can be disorienting, cause problems with depth perception, and might even cause nausea.
Here is a detailed guide on how to measure your IOD using a ruler and a mirror: How to Measure your IPD. (Yes, I'm aware I'm calling it IPD in that article.)
Once you know and have memorized your IOD, you should dial it in after putting on the headset. There is a small knob on the lower-right hand side of the headset; twist that knob to change the physical separation between the left and right lenses, and the IOD configured in the VR software. The VR software will show you the current IOD value in a small pop-up window when you twist the knob. Twist until the number matches your real IOD as closely as you can get it. Do not continue twisting the knob if you feel resistance. There is a maximum IOD adjustment range; if your IOD is outside that range, you're a little bit out of luck.
In general, if you need prescription eyewear to see clearly at a distance of around 2m (6.5 feet), you will probably want to wear your eyewear while wearing the Vive VR headset. While the headset's screens are very close to your eyes (about 2-3cm), the imagery you see in the headset is optically about 2m away (here's why).
When wearing eyewear, follow the proper procedure for putting on the VR headset to avoid knocking off your glasses, and please be careful not to scratch the headset's lenses with your glasses. They are plastic, scratch easily, and cannot be replaced. If you have a hard time fitting your glasses inside the headset, there are buttons on the side to make additional space between your eyes and the lenses.