The sad truth is that companies that give lifetime warranties sometimes go out of business. That means that the "lifetime" they were talking about was theirs, not yours. In our time in business we've seen many online companies as well as manufacturers go out of business. In these tough economic times we're likely to see more.
If you're buying from a big chain they often change manufacturers more often than some people change their shirt. That means that if someone bumps into your fence with a car or it's otherwise damaged you can't get replacement parts when you need them. Even worse is if the fence had a manufacturing defect and it actually failed. This can happen if the initial "recipe" for the material was not right or poorly mixed during the extrusion process. The good news is that it doesn't happen very often and with the good companies almost never. More good news is that if a vinyl fence were to fail it is most likely to happen or show up fairly quickly.
The window is from the time of installation to a few years out buy not likely more than five. If the manufacturer you've chosen is still in business five years from the time you bought your fence then you're probably "out of the woods" and your fence will last as long as you do. If the manufacturer has been around a long time (like Certainteed who's been around over 100 years now or Bufftech who's been in the vinyl fence business since it started close to 25 years ago) then you're probably in good shape.
Another important question about the warranty is "If the fence fails, does the warranty cover labor to take out the old fence and install a new one"? Many do not. Another consideration is that if the manufacturer for your fence is a foreign company will they still be exporting here when you need them. If they're half way round the world it will be pretty hard to get them to honor their warranty if they don't want to. This is particularly important if you were dealing with a big chains or home improvement stores and they have switched manufacturers. In that case finding the manufacturer to back up the warranty may be very hard to do.
We send you a sticker to put on your fence with our 800 phone number so if there ever is a problem you won't have to sort through old paper work to get a hold of us for your warranty. Additionally, we have been able to help customers with claims even when the claim wasn't covered under the warranty so we'll be there to help if ever needed.