VGA was a huge and very bold rejection of NTSC!
TLDR of this whole history section: VGA and NTSC have nothing to do with each other. VGA was a blank-sheet design that smashed NTSC limitations with extreme prejudice - and planned to never, ever, ever go back.
Apple IIc monitors, like most monitors of that age, use the NTSC composite video standard in all respects that are not incompatible with their purpose. For instance monochrome displays do not use the color modulation obviously. S-video displays deliver luma and chroma separately. RGB delivers R G and B as three separate monochrome channels. Other than that, they are NTSC through and through.
These were tentative tiptoes away from traditional NTSC; they stayed with the standard in all other respects - frame rate, scan line rate, timing and screen dimensions. This frustratingly limited screen resolution, right up until VGA finally smashed the "glass ceiling": it threw away NTSC altogether and started with a blank-sheet new design. The word was "we will no longer support using your TV as a display device; get a dedicated PC monitor".
And THAT required the market to shift from "basically everyone using TVs as their display" in 1977 to "almost everyone using a dedicated PC monitor" - and remember at the time, monochrome monitors were $200-300 and color monitors were $700. Even in the mid-1980s, the need to drive a TV had strong sway - can you imagine the PCjr shipping with no ability to connect it to a TV? Impossible!
The only company ever to get away with smashing the NTSC monolith was IBM, with their niche Monochrome Display Adapter (MGA) in 1981 (~350 scan lines per field instead of the usual 200).
So again it was IBM to introduce original VGA (640x480 non-interlaced per field), with their meant-to-be-revolutionary Personal System/2 (PS/2) line. IBM was correct that nobody in the business world cared about TV support and would not object if asked to pay $700 for a dedicated-to-purpose color monitor. The goal of VGA was to entirely free computer displays from the albatross of NTSC limitations. No NTSC support was even imagined; the goal was to leave NTSC in the dust.
*But mind you, 1987 was also when the Amiga was starting to "find its feet" in the marketplace. Heck, Amiga's Video Toaster launched in 1991 and wound up becoming the king of midsize TV production for the whole decade - a lot of local TV markets used the Video Toaster for graphics and fonting of news, commercial production - Amigas even did the CGI on Babylon-5.
But the market message was clear: All-in, or all-out of NTSC. No more "stuck in the middle", no more "NTSC albatross dragging down PC resolutions".
It isn't resistors
As such, there are only 2 ways to get from a VGA output to an NTSC-family display:
- Use a frame buffer
- Have the video card be versatile enough that you can change its parameters to actually output NTSC timing signals, even though they will be RGB of course. From that point it's simply a matter of mixing the sync and luminance signals correctly, and yeah, that you can do with analog circuitry... but only if the video card is able to do all the heavy lifting because its designers were extra clever and provisioned the ability to do that weird old thing.
Really, you're better off starting from scratch
The right way to solve this problem, really, is to have a USB or Lightning attached external GPU that inherently outputs NTSC composite/EGA/TARGA-24. StackExchange is not a product-recommendation site, but I gotta imagine somebody makes or made such a thing, for old-school TV production if nothing else. Technologically, it would be little more than a dongle. Far cry from the stacked/stuffed TARGA-24 boards!
Then, appropriate video drivers so your Linux/Windows/Mac system can see it as "just another display" like it does with other GPUs.
This GPU certainly could maintain a 533++ x 400 internal buffer. With monochrome NTSC displays, there is no inherent limit to the number of horizontal lines, depending on how you feel about pixels being square. And if the display supports interlacing, you can also get 400 vertical resolution, although picture elements would need to be at least 2px wide (so they are present on both fields) or they would flicker.
Keeping in mind that most NTSC-era computers outputted a non-interlaced signal 200 pixels tall x 280-320 horizontal. Interlaced TVs were able to tolerate this. Most likely, your Apple IIc monitor has the same vertical sweep hardware as other TVs of the day, and would accept interlacing.