Very high-resolution (R ~ 110 000) spectroscopic observations of the very low-mass star VB 8, obtained with the Utrecht Echelle spectrograph (UES), are presented. A flare occurred during the third of a series of four consecutive spectra. This event produced significant enhancement in the strength of the emission lines of He i D3 and Hα in VB 8, as well as a filling up of some but not all atomic absorption lines, and a weakening of the γ TiO bandheads at 705.4 and 712.5 nm. Core emission in the Na i D1, D2 and K i 769.9-nm resonance lines also became stronger during the flare. Other very low-mass stars observed with the same instrumental configuration include GJ 406 and VB 10. The UES spectrum of VB 8 is compared with these. In particular, VB 10 appears to have a lower gravity than VB 8, indicating a lower age and a smaller mass which could be very close to the substellar mass limit for solar metallicity.