That VSC is a video scan converter = a scaler, in this case a downscaler. The specs on Extron's website specifically state that it will take "Horizontal frequency .... Autoscan 24 kHz to 70 kHz" input, so it won't take 15kHz (240p/480i) input. It's designed for you to feed in 24kHz/384p or more likely 31kHz/480p input and above up to about 1024p, and will output at 15kHz 480i in the signal types it lists (Componnet, S-video, etc).
As far as downscalers go, the Extron VSC 150 is going to be very nice. It’s analog-to-analog by the best known manufacturer out there, so it’ll be fast and pretty clean. It looks like it has a few different filtering options for the downscaling. But it’s still a downscaler. So you lose image quality, and you only get 480i.
The HDMI options will be the same but worse. They won’t take 15kHz input (generally), and will output 480i probably in a single signal type. They will lose image quality and only give 480i. But they will also lag, and the cheaper the converter the more lag you’ll get. Some people don’t notice much, but some people go crazy.
If your Nvidia 750ti is capable of outputting 15kHz video over the DVI-I port, then a passive DVI-to-VGA dongle and a VGA-to-Component transcoder capable of 1:1 conversion is what you’re after. When you feed your game signal from mame at, say, 256x224@59.97Hz into one of these units, you get precisely the same out of it, it’s not forcing an output mode. The only ones that take VGA input that I can think of are the Behar Bros Garo, and the retrotek one on ebay. The new version of the Garo seems to be having issues though. You should hunt around on the Shmups hardware forum, there's usually a fair bit of talk about converters on there.
Just make certain your GPU is outputting 15kHz before buying, of course
TBH if the extron VSC 150 is cheap, like $30 or less shipped, i'd give it a look. But i'm the type of person who would try to get like a 320x480p mode into it to see what happened
For a more reliable solution, you can look at a cheap Radeon card and crt_emudriver, which will give you composite sync, which lets you use a VGA-to-SCART cable even if that cable says it's not for PC. The SCART cable gives you access to a few more transcoders like this one which is meant to be ok -
https://www.ebay.com/itm/NEW-SCART-RGB-to-YUV-Component-Video-Converter-Scaler-/221156873851 If you can cut/wire/solder, one you have composite sync you could go something like a Jrok converter, or the cheaper probably-good-enough timeharvest unit -
https://www.aliexpress.com/store/product/Arcade-RGB-CGA-to-TV-and-Video-Composite-converting-board-game-accessory-for-arcade-game-machine/607318_502847877.html?spm=2114.12010612.8148356.8.f5295c2aZXVAMz