In the 3.15 release repo below, S4Vectors is missing:

https://www.bioconductor.org/packages/3.15/bioc/src/contrib/Archive/S4Vectors/

How can I download S4Vectors version 0.34.0?

It is missing because that package did not have any updates in the release except for the initial version. The archive directory holds previous versions when changes are pushed to a release. The tar is available in the main source repository. Are you not using BiocManager to handle downloading the proper versions? It should grab 0.34.0 automatically?