Potassium (K) is present in soils mainly in minerals, including feldspar. However, most of it is unavailable to plants. In the in-dyked alluvial soils of the Mekong Delta, available K is typically low despite the abundance of K-bearing feldspar, leading to nutrient imbalances and yield constraints. This study aimed to (i) select potential feldspar-potassium-solubilizing purple nonsulfur bacteria (K-PNSB), (ii) determine their ability to enhance hybrid maize seed vigor (Zea mays L.), and (iii) evaluate their effects on the growth of maize seedlings. Fifty-eight K-PNSB strains were isolated from maize-cultivated in-dyked alluvial soils, with soluble K concentrations ranging from 0.108 to 15.0 mg L−1. Among these, strain M-Sl-03 released the highest K concentration under microaerobic light conditions, whereas strains M-Sl-01 and M-Sl-06 produced best under aerobic dark conditions. In addition, two more strains, M-Sl-02 and M-Wa-06, were also selected for their K solubilization ability. The selected strains were identified as Cereibacter sphaeroides strains M-Sl-01 and M-Sl-02, Rhodopseudomonas palustris strain M-Sl-03, and Rhodoplanes pokkaliisoli strains M-Sl-03 and M-Wa-06, according to their 16S rDNA region. None of them exhibited toxicity to germinating maize seeds. Both individual strains and the five-strain mixture significantly improved seed vigor. At a 1:1000 dilution, individual and mixed inoculants increased the vigor index of maize seeds by 47.5–68.8%. In addition, the selected PNSB strains contributed to improving the growth of maize seedlings, particularly plant height and root dry biomass. These promising strains have potential for application as biofertilizers to support hybrid maize cultivation.