One thing I see a lot of in old/retro/vintage/prehistoric computing circles would have to be confusion over what is, and what is not, considered to be vintage. Some people claim anything from a Pentium 4 on down is vintage. But on the other end of the spectrum, we've got people saying if it ain't a 486 or older, it's just old.
Since these opinions appear to be so omnipresent in these circles, I thought I'd take a moment (no, really, a moment -- it's 3:15 in the morning and I have things to be doing, although nothing involving bedtime!) to just toss my opinions out there.
So, just answer it already, are Pentium class systems vintage?
Yes, they are.
And to be 100% upfront with you, I don't even think this one is a matter of opinion. The P5 microarchitecture on which the Pentium is based, is nearing 30 years old. Pentium systems aren't capable of doing anything modern (that's not to say they can't do it with some shoehorning), although basic tasks can be done. This entire page was actually written on a PC with a 75 MHz Pentium processor.
So yes, they are absolutely what I (and probably many others, but not some!) would consider to be vintage systems. If you are reading this article because you have a Pentium system you're looking to sell, this does not mean that you should charge a hefty premium (~$100-150 on up for just the PC by itself) for it on eBay or other sites!!!
And, when it comes to my personal opinion, anything Pentium II (possibly even III in short time) or older is what I'd call vintage. That doesn't mean I don't find newer systems useless, though. Pentium 4 systems hold a good deal of nostalgia for me, which on its own is enough to draw me to use them. But if you play your cards right, they can be very useful for small tasks, such as web or file servers, or perhaps even very light web browsing under a lightweight operating system (although I would not recommend this).