As far as I can tell, Bambusa wamin is the cultivar of Bambusa vulgaris. I am basing this on first its absence from the latest work on Bambusoideae phylogeny and other papers, like the Chung et al paper (linked), stating it to be "Bambusa vulgaris cv wamin."
Cheers, Jen
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The name Buddha Belly Bamboo is actually more frequently used for Bambusa ventricosa, a species from China much used in Penjing/Bonsai, but also for Bambusa vulgaris 'Wamin', which is how that cultivar name should be written. Because the latter is only known in cultivation, and not in the wild, it is generally considered a cultivar. In my opinion it is not sufficiently different from the type of Bambusa vulgaris to be considered a separate species or variety anyway, but that is a subjective opinion, and the names Bambusa wamin Camus, B. vulgaris var constrictinoda Proudloch and B. vulgaris 'Wamin' are probably all synonymous and are available for use if anyone wishes to consider it to be a separate species or variety rather than a cultivar. The name Bambusa vulgaris var wamin does not seem to have been validly published.
Also you can look that different sources provides quite different information. www.theplantlist.org aon www.catalogueoflife.org. gives the same information that Bambusa vulgaris f. waminii T.H.Wen mentioned as a synonim to Bambusa vulgaris Schrad. But www.ipni.org. give us information that Bambusa vulgaris Schrad. ex J.C.Wendl. f. waminii T.H.Wen is valid forma according to Index Kewensis