Agave angustifolia needs to be separated out from Agave vivipara as its own species. According recent articles by AR Frank (2012) and A García-Mendoza & F Chiang (2003) as well as a book by AG Valensuela Zapata & GP Nabhan (2003), it has now been established that A. vivipara L. does not have priority over A. angostifolia Haw. and that they are two separate species. From a google scholar search of literature since 2003 seems to be mostly accepted in the biomolecular and ecology literature that angustifolia is a distinct species. AG Valensuela Zapata & GP Nabhan (2003) stated in their book it has been generally accepted that A. vivipara L. does have priority over A. cantala.
Frank, Alan R (2012). Guide to Agave, Cinnamomum, Corymbia, Eucalyptus, Pandanus and Sanservieria in the Flora of Floria. Phytoneuron 2012-102:1-23
Garcia-Mendoza, Abisai & Fernando Chiang (2003). The Confusion over Agave vivipara L. and A. angustifolia Haw., two distinct taxa. Brittonia 55(1) 2003: 82-87
Valensuela Zapata, Ana Guadalupe & Gary Paul Nabhan (2003). Tequila: A Natural and Cultural History. University of Arizona Press.
Simpson, June et al. (2011) "Genomic resources and transcriptome mining in Agave tequilana" GCB Bioenergy 3(1) 2011:25-36.