The taxonomy of Zimmerius vilissimussensu lato varies among authorities, recognizing anywhere from one (Traylor 1979), two (Fitzpatrick 2004), three (Dickinson and Christidis 2014), or four species (del Hoyo and Collar 2016). Rheindt et al. (2013) demonstrated that there are at least three non-sister taxa under this name, and suggested several taxonomic adjustments. Their data were less clear on the relationships between parvus and nominate vilissimus (Guatemalan Tyrannulet), sampling just one individual of each taxon and finding that they are in a clade with Zimmerius albigularis(Choco Tyrannulet), and may or may not be sister taxa. Additional sampling is needed, but many authorities recognize vilissimus and parvus taxa as separate species, based on differences in plumage, vocalizations, and habitat.
Jones, A. W. (2018). Mistletoe Tyrannulet (Zimmerius parvus), version 2.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.paltyr3.02