Hi,
Thank you for the details, it helps me a lot.
While it is a bit hard to tell by only looking at the front-end, I believe you need to enable and change a few features, which may help:
– Make sure to enable the Search in terms option: https://i.imgur.com/VsIWd8y.png
This will make sure, that the product manufacturers are taken into consideration.
– Change the behavior options like so: https://i.imgur.com/YzRPFz8.png
While it is usually not recommended to use a word length of 1, in this case it may help, as there are lot of vitamin names, where even 1 letter is significant.
These changes should make a significant difference in results.