Warning: Opinions ahead.
It is easiest to just pick a cut-off date and be done with it. For me, anything 20 years old or older is vintage, full stop. No debating, no arguments; most non-collectors have no trouble understanding this. As time marches onward, so does the year something can be considered vintage. In the year 2020, systems from Y2K will be considered vintage (because they will be!).
I don't think it is useful to define "eras" because there are always outliers that get people arguing. For example, here's a few that are hard to classify (and no, I am not looking for anyone to start arguing about these):
- The IBM PC is a 16-bit CPU with an 8-bit data bus. So is it a 16-bit system or an 8-bit system?
- The TurboGrafx-16 is a console with an 8-bit CPU and a 16-bit graphics processor. So does it belong to the 8-bit console "era" (NES, Colecovision, SMS, etc.) or 16-bit "era" (SNES, Genesis, etc.)?
- The Atari Lynx is a hand-held console with an 8-bit CPU, a 16-bit graphics process, and a 32-bit math co-processor (multiplies can have 32-bit result, can divide 32-bit numbers). So where does it belong?
- The Atari Jaguar is a console with a 16-bit general-purpose CPU, two 32-bit RISC CPUs, a 64-bit graphics processor, and 64-bit bus. So...?
So defining hard year cut-offs for these things never work well, that's too simple. I guess you could define overlapping ranges, sure, but not hard cut-offs.