1 INTRODUCTION
2 DATA AND SAMPLE PROPERTIES
2.1 Rest-frame UV VANDELS sample and observations
2.2 Rest-frame Optical MOSFIRE sample and observations
2.3 Measurements and derived quantities
2.4 The ${\bf z}\sim3.4$ NIRVANDELS sample
3 DETERMINATION OF GALAXY METALLICITIES
3.1 Determination of the gas metallicity $(\mathbf{Z}_{\mathrm{g}})$
3.2 Determination of the stellar metallicity $(\mathbf{Z}_{\star})$
4 RESULTS
4.1 The gas-phase mass-metallicity relation at ${\bf z}\simeq3.4$
4.2 The stellar mass-metallicity relation at ${\bf z}\simeq3.4$
4.3 Enhanced O/Fe ratios at high redshift
5 DISCUSSION
5.1 Abundance patterns compared to Milky Way stars
5.2 Excitation conditions at high redshift
6 SUMMARY AND CONCLUSIONS