Lets start with what chamber music is (and isn't).
Essentially, chamber music describes music written for one or more performers. An ensemble that could fit comfortably in a living room or a salon with room for a small audience. In a time before electricity (and TV, radio, and Netflix), chamber music was really entertainment for a house party. They could be informal get-togethers with amateur musicians sharing the enjoyment of music with one another, or more formal gatherings with professional musicians doing the honours.
But chamber music is quite different from a singalong around a piano. Each instrument or voice has its own individual musical part and no one part is more important than any other. Chamber music is really a conversation in which musical ideas are presented, exchanged and discussed among the performers, each having a say.
This makes chamber music quite distinct from orchestral or choral music which have several performers playing or singing the same part.
The most popular form of vocal chamber music is the "art song" or "lied" (pl. "lieder") which came into prominence during the romantic period. The poems of writers, such as Heine and Goethe were set to music by Schubert, Schumann, Brahms, and others for voice and piano. In lieder the piano is an equal partner with the voice rather than just accompaniment. The piano part can establish the mood, set up the action and reflect musically the meaning of the text.
Schubert's famous musical telling of the Erlking fable is an excellent example of art song and vocal chamber music....