In IUCAT in the Advanced search, search for the subject heading "vocal music bibliography." Make sure you select "subject" from the drop-down list!
To be taken to the results of this IUCAT search, simply click this: https://tinyurl.com/svwfmuv4
You can also browse vocal repertoire guides in person. Most are in the reference section, but some are on the 3rd floor.
Begin with the call numbers: ML128.V7 and ML128.S3
You may also visit Song Helix to search for repertoire by category or attribute.