Cypripedium calceolus L. (European yellow lady's slipper) is one of the rarest and most vulnerable members of the family Orchidaceae in river-terrace forest communities of the southern taiga subzone (Slobodskoy District, Kirov Region). The species is included in the Red Data Book of the Russian Federation and the Red Data Book of the Kirov Region. The paper presents the results of a comparative analysis of the variability and phytocоenotic plasticity of morphometric parameters. The vitality structure dynamics of 7 cоenopopulations (CPs) of C. calceolus were determined over a 3-year observation period (2021–2023). It has been established that the weather conditions during the current growing season have a statistically significant effect on most of the plants’ morphological parameters except for the number of flowers. The scope of variability in the morphological parameters of C. calceolus in the study area varies widely, from low to high, but the average level remains the most characteristic (predominant) in the dynamics. The highest phytocоenotic plasticity was exhibited by the aboveground shoot height and the flower staminode width. The lowest plasticity was observed for the number of flowers and leaves, as well as for the dimensions of the labellum and perianth. The vitality structure of the studied C. calceolus CPs in the southern taiga subzone during the study period was dominated by prosperous vitality types, but in some years CPs 2.2a, 5, and 6a showed depression signs. The vitality index (IVC) was found to depend on the season-averaged precipitation in the current growing season.