Is it possible that there are unknown vitamins?
Yes, it is possible that there is a vitamin or other essential nutrient that has not been identified so far.

Essential nutrient means any substance normally consumed as a constituent of food which is needed for growth and development and/or the maintenance of life and which cannot be synthesized in adequate amounts by the body (fao.org). Essential nutrients include vitamins and some minerals, fatty and amino acids.

Choline: An Essential Nutrient for Public Health (PubMed, 2009):


  Choline was officially recognized as an essential nutrient by the
  Institute of Medicine (IOM) in 1998.


The fact that choline has been recognized an essential nutrient relatively late suggests that there may be other nutrients that are not recognized as essential today but may be in the future, for example:


Aluminum (Biocyclopedia)
Boron (PubMed)
Nickel (WebMD)
Silicon (PubMed)
Vanadium (PubMed)