Hi David,
You're right, in the Term field you would add Plasmodium vivax. In the Vocabulary field, MeSH appears to be what's entered most often.
Dataverse's
Vocabulary URL field is taken from the DDI Codebook 2.5 schema, where it "specifies the location for the full controlled vocabulary." I don't have experience with the development of Codebook 2.5, but maybe this was created at a time when it wasn't common for vocabulary terms to each have their own URIs? In any case, the need for Dataverse to be able to record the term URI (either as something the data depositor enters or as something that Dataverse automatically associates with a term that the depositor has chosen from a list of terms) has been brought up during Dataverse community conversations (as well as in a DDI Codebook working group).