This article reports the luminescence properties of amphipathic YVO4:Er3+/Yb3+ nanoparticles (average grain size ca. 20 nm) obtained by an oleate-aided hydrothermal process. Depending on the upconversion (UPC) and downconversion (DWC) processes, they show luminescence in the visible and near-infrared (NIR) regions, respectively, by 980-nm excitation. The sample doped with Er3+:2.5 mol% and Yb3+:10 mol% showed the highest luminescence intensity in both the visible and NIR regions as a result of efficient energy transfer from Yb3+ to Er3+ ions. The hydrothermal treatment greatly enhanced both the DWC and UPC luminescence efficiencies. This is due to the reduction in the concentration of surface defects and ligands, accompanied by grain growth. NIR Fluorescence microscopy revealed for the first time that DWC luminescence is sufficiently intense for application of these nanocrystals as a NIR bioprobe.