Optical diagnostic diagrams for our sample. Left: Classical BPT diagram (Baldwin et al. 1981) for our subsample at z < 3, color-coded by EW(CIII]). The dashed black lines are the typical local AGN/SF demarcation lines (Kewley et al. 2001; Kauffmann et al. 2003). The dashed red line is the demarcation at z ∼ 3 according to Kewley et al. (2013). Right: Mass-excitation diagram for our entire sample color-coded by EW(CIII]). The dashed black and red lines are the AGN/SF demarcation at low-z (Juneau et al. 2014) and z ∼ 2.3 (Coil et al. 2015), respectively. As a reference, we include the stacks from Cullen et al. (2021). Our VANDELS subsample is a subset of these stacks. In both panels, the symbols with magenta edges are galaxies classified as AGN according to the UV diagnostic diagrams in Fig. 6.