Note that pgvector isn't supported on any of the large cloud providers' hosted Postgres offerings, other than Supabase. https://github.com/pgvector/pgvector#hosted-postgres has instructions on how to add your voice to request it to be added!

(It does seem that the ancient https://github.com/eulerto/pg_similarity is supported by RDS and Google Cloud - but it's hard to tell whether attention was paid to its performance characteristics with nearly the rigor that pgvector seems to have been designed.)