Hexagonal-phase NaYF₄ (β-NaYF₄) has been acknowledged to be one of the most efficient doping hosts to prepare bright lanthanide-doped luminescent nano-bioprobes for various biomedical applications. However, to date, it remains a great challenge to synthesize ultra-bright lanthanide-doped β-NaYF₄ nano-bioprobes under a low reaction temperature by using conventional synthetic methods. Herein, we first develop an acetic acid (HAc)-mediated coprecipitation method for the preparation of ultra-bright lanthanide-doped β-NaYF₄ nanoprobes under a low reaction temperature at 200 °C. Based on a series of comparative spectroscopic investigations, we show that the use of HAc in the reaction environment can not only promote the rapid α–β phase transformation of NaYF₄ host at 200 °C within 1 h but also boost the absolute photoluminescence quantum yield (PLQY) of NaYF₄ nanocrystals to 30.68% for near-infrared emission and to 3.79% for upconversion luminescence, both of which are amongst the highest values for diverse lanthanide-doped luminescent nanocrystals ever reported. By virtue of their superior near-infrared luminescence, we achieve optical-guided dynamic vasculature imaging in vivo of the whole body at a high spatial resolution (23.8 µm) under 980 nm excitation, indicating its potential for the diagnosis and treatment evaluation of vasculature-related diseases.