Environmentally friendly extraction technologies for biologically active substances (BASs) are an actively developing and important industry. In recent years, the development of this area has been associated with the use of low-melting mixtures, which are most often referred to as “deep eutectic solvents”. Vaccinium vitis-idaea L. is a valuable source of phenolic biologically active compounds. However, to date, there are limited studies devoted to the use of such solvents for the extraction of biologically active substances from V. vitis-idaea. This study introduces the use of low-melting mixtures of choline chloride or betaine with lactic acid and water for the ultrasonic extraction of phenolic secondary metabolites from V. vitis-idaea leaves for the first time. The kinetics of extraction have been studied, and the extraction conditions have been optimized using a Box–Behnken design. It was found that the optimal extraction conditions are follow: the most suitable mixture is betaine with lactic acid and water at a molar ratio of 1:10:5, the optimal temperature is 33 °C, and the optimal ratio of the mass of plant material to the volume of the solvent is 1:20. Under these conditions, the yield of total phenolic compounds was 744.3 ± 1.2 mg GAE/g, and total flavonoids reached 24.4 ± 0.2 mg RE/g. The IC50 values of the obtained extract were 2.45 mg/mL for free radical scavenging with DPPH and 3.47 mg/mL for ABTS. The data obtained can be used in the development of green technologies for the extraction of biologically active substances from the leaves of V. vitis-idaea.