This is indeed tricky – because the title does not actually start with “viatmin a”, and the keyword “a” is simply too broad for a match.
Let’s try something different here.
On the index table settings make sure the following options are set:
– For the index product categories are selected: https://i.imgur.com/7WHS2hL.png
– The keyword length is set to 1 and stop words are disabled: https://i.imgur.com/vE3CRR4.png
Now, even if you don’t change anything, create a new index anyways: https://i.imgur.com/GasZcMn.png
For the search settings, turn off taxonomy term results for now: https://i.imgur.com/2LqhksS.png
Now, the product categories with the exact keyword match are in the system now. Let’s try to prioritize the term matches, by significantly changing the weights for the index matches like this: https://i.imgur.com/TuQGq2x.png
For starters use the value 99999, then you can lower it afterwards.
If all goes right, this should put the matches based on the product category to the top. That means typing “vitamin a” should find products categorized “vitamin a” and give that match a super high relevance value, (hopefully) putting it to the top.