I'm not one to get caught up much in these sorta semantics, but I tend to consider "retro" to be more along the lines of modern products designed after older ones, while "vintage" would refer to the original old products in question.
For instance, I saw some retro-looking cheap audio equipment in the local Target, styled after radios of decades past, but I'm pretty sure the construction would be plastic with modern solid-state ICs all over the place.
By contrast, there's this Yamaha CR-1020 I scored at the local Goodwill not that long ago. Wooden cabinet with metal vents, faceplate and smooth, perfectly-damped knobs, and almost no ICs to speak of under the hood; unquestionably 70s vintage in terms of its build quality and sound of a time when the home stereo wars were in full swing, before the great "black plastic crap" malaise of the 1980s and later.
As for where I draw the line with computers, I tend to group it by periods of OS support and compatibility, and they don't always line up cleanly depending on which computers we're talking about. I tend to focus a bit more on software/OS, as the hardware is ultimately nothing without that.
IBM-compatible: The biggest divider to me is the Win9x and 2000/XP divide, since going to NT-derived versions like the latter break so many 9x-era games it's ridiculous.
I built a computer specifically to straddle the line here, running a Pentium 4 EE 3.2 GHz, 2 GB of DDR-400 and a GeForce 6800 Ultra to power through XP-era games up through 2005 or so, but also capable of running most Win9x games with little fuss (sometimes requiring CPU underclocking here and there) and also packing one fully-functional ISA slot for sound card use, courtesy of its industrial motherboard. It might just be powerful enough to use as a basic computer if my more modern machines go down, though I certainly wouldn't want to.
Macintosh: Classic Mac OS/Mac OS X, obviously. Again, I have a MDD Power Mac G4 1.42 GHz specifically to straddle the line here, as it can manhandle anything under OS 9 while running OS X Leopard semi-decently. However, there's the occasional old game that requires me to use a Mac version of the Thrustmaster FCS to get the best control out of it, so I need something with native ADB, like my Power Mac 6500. (Ideally, it'd be a 9600 or B&W G3, but that'll have to wait for another time.)
PowerPC systems are all but useless for modern computing these days, though. OS X PowerPC-compatible software is too difficult to find; OS 9 and prior is ironically much easier due to certain sites. TenFourFox can get you on the modern Web, but YouTube is a slideshow on 1.42 GHz G4s. I pretty much just keep mine around for Mac-exclusive games.
Amiga: 68k/PowerPC, since any given AmigaOS version tends to only run on one or the other. Also, I don't really consider AmigaOnes and the other ones produced afterward to be "true" Amigas, as they lack the custom chipset inherent to the 68k ones and rely entirely on emulation.
With all that said, one thing I have noticed post-Core 2 is that even ten-year-old computers like my Q6600 build (albeit spruced up with 8 GB of DDR2-800 and a GTX 760 compared to its initial 2 GB and 8800 GT) are still viable today, given how my little bro still enjoys PC gaming on it and it still holds its own for a lot of recent games. My 4770K/32 GB DDR3-2400/GTX 980 build runs circles around it, obviously, but that doesn't make the old system unusably slow for anything but the most demanding of games.
That would not have happened at all pre-Core 2, as hardware was advancing too fast and games were too eager to take advantage of the increases in power to the point where you'd have to build a whole new computer in the very next year or two just to keep up because of the required new motherboards to back up those new CPUs. Maybe there will come a time that "vintage" isn't effectively useless for everyday use - well, at least if we don't run into any more Meltdown/Spectre wake-up calls that require new hardware to fix.