There are probably more performant options, but for universality, you are best off going with a popular SQL DB that has vector extensions or support. SQLLite is another option if you want even more portability.
pgvector is very performant for tiny scale, especially if they don't expect to update the knowledge base constantly and don't care about millisecond differences in speed. It's also probably the only major vector db with ACID. And it's substantially cheaper than many others.
I haven't looked into SQLite and such, but many of the other SQL type of options were not good last I checked. And it takes quite a bit of effort to dig through them because all of these will show research that puts themselves as the best lol.
There are probably more performant options, but for universality, you are best off going with a popular SQL DB that has vector extensions or support. SQLLite is another option if you want even more portability.