Madagascar periwinkle may prefer sandy soil at low altitudes but is capable of surviving in a variety of tropical and subtropical habitats, including in disturbed areas.  It is self-compatible, and has spread from cultivation to become naturalized in many parts of the world.  Some therefore consider the plant to be an invasive weed.  Although it does readily escape from cultivation, it does not proliferate to the point of overgrowing and wiping out native vegetation in areas where it becomes established, as do seriously invasive plants such as kudzu or Japanese knotweed.  Its introduction therefore probably does not cause significant harm to most local ecosystems.

References

Catharanthus weediness risk assessment for Hawai’i (with useful references)

van Bergen, M. A. 1996. Revision of Catharanthus G. Don. Series of revisions of Apocynaceae XLI. Wageningen Agricultural University Papers 96-3: 9-46.