D. vinealis is over-recorded, especially away from the warm southern parts of Britain, which are its stronghold. Many bryologists find Didymodon tricky, with good reason, and the acute-leaved species that often grow on the ground (such as D. fallax, D. insulanus and D. rigidulus) are easy to confuse with each other and D. vinealis.
Some understanding of these species is gained by being aware of the unifying characters of the different Didymodon sections (taxonomic groups). D. vinealis is a member of section Vineales, which also includes D. nicholsonii and D. insulanus. Serious taxonomic study into delimitation of its species is needed and what we currently call D. vinealis is probably an aggregate of several cryptic species, which may help to explain some of its considerable variability. Further useful information is available from the Resources page.
A good clue to D. vinealis in the field in the way that dry shoots have upper leaves that are slightly twisted, in contrast with the strongly curled and twisted dry shoots of D. insulanus. However, in that respect it may resemble D. rigidulus. If you carefully slice off the upper 1/5 of a number of stem leaves and float them in water under a coverslip you’ll find that at least one orients itself so that you can view the upper (ventral) side of the lamina and nerve. Just below the leaf apex, the nerve appears to lie in a channel, the so-called ventral groove, and moreover has enlarged translucent cells in the base of the channel (see Claire Halpin’s excellent images in the gallery). Lookalike species that are not in section Vineales lack this.Read the Field Guide account