Depending on how 'one-off' the query is, sequential read is the right answer. The alternative is indexing the data for ANN, which will generally require doing the equivalent of many queries across the dataset.

On the bright side, smart folks have already thought pretty hard about this. In my work, I ended up picking usearch for large-scale vector storage and ANN search. It's plenty fast and is happy working with vectors on disk - solutions which are /purely/ concerned with latency often don't include support for vectors on disk, which forces you into using a hell of a lot of RAM.

https://github.com/unum-cloud/USearch