While commercial vaporizers use it, I don't believe that PVC is safe enough for this purpose. Don't use it. As you can see I I've read about leeching in medical products (while they were using specific chemicals), and also read that the PVC should not be heated over 180F which is not enough. Silicone tubing can be bought online. I use thin copper tubing.
Where I live smoking tobacco is restricted to people 18 and over. If your under 18 don't make this, don't smoke, or vaporize, or whatever. While vaporization may reduce the health hazards associated with inhaling plant material, I'm certain that it does not completely eliminate them. Don't smoke. It's addictive and bad for your health. Depending on what region or country you live in, it may be illegal to consume specific herbs. Make sure that the herbal blend you choose is legal in your region. This guide is not intended to assist in the illegal (And highly immoral) consumption of banned plant materials.
This vaporizer is a fire hazard. It's cheap and easy to make, but there's a trade off. This one cost me about $25. A similar commercial vaporizer would cost 150-250$.
You might consider buying one of those wall timers so that if you leave your vaporizer plugged in by accident it will only run as long as you set the timer for. This is a must if you plan on vaporizing any herbs which may induce forgetfulness. Don't start a fire!
Further I've been warned that copper fumes are vary dangerous. I don't think copper will fume at the temperatures I'm subjecting it to.
That said the consensus among many smokers is that the most sure healthiest thing to smoke out of is glass. If you find a good way to do this with glass, more power to you! For now my brass on glass design might be the best DIYers can hope for.
One more warning: Don't use a soldering iron that has come anywhere near solder. Solder is super poisonous nasty stuff. Just buy a new one. They are really not that expensive.