Catharanthus roseus (L.) G.Don var. albus

See Catharanthus roseus (L.) G.Don.  Catharanthus roseus var. albus seems the most likely identification because Vinca alba was frequently described in the Victorian literature as a tender plant, along with Vinca rosea and another form, Vinca alba occulata, white with a red centre.  The following is from a discussion of Stove Vincas, making it quite clear that tender plants are under discussion.  ‘These old inhabitants of our stoves are neglected plants in many gardens, and in the majority entirely discarded.  It is to be regretted that such good and useful plants are cast aside to make room for others of perhaps less beauty and use.  Few plants are easier grown, or attain a good size quicker, if subjected to liberal treatment.  All the stove varieties make capital plants for exhibition purposes, and look magnificent amongst a collection of flowering-plants, when well grown and neatly trained.  They are even more striking in a collection of plants, especially V. alba and V. occulata, than many other plants grown for the purpose and exhibited. […] Vinca alba and its red-eyed variety V. occulata are the two most worthy of being grown.  V. rosea is not so serviceable, and its flowers are not very brilliant in colour.’ [The Gardener vol.15, p.392/1881].