Background/Objectives: Biosecurity and stamping out are key control measures against H5 high-pathogenicity avian influenza (HPAI) outbreaks. Vaccination in poultry is an additional tool to reduce disease risk and facilitate timely containment. This study aimed to establish a candidate vaccine strain against H5 HPAI in Asia and validate its protective efficacy. Methods: Based on genetic and antigenic analyses, a representative HPAI virus, A/duck/Vietnam/HU16-DD3/2023 (H5N1), collected in northern Vietnam, was selected to generate a candidate vaccine strain, rgPR8/VN23HA?KRRK-NA (rgPR8/VN23; H5N1), using reverse genetics, followed by formulation of an inactivated oil-adjuvanted vaccine. Vaccine efficacy was evaluated by measuring humoral antibody responses after intramuscular vaccination and by assessing mortality and virus recovery following intranasal challenge with a clade 2.3.4.4b virus, A/Ezo red fox/Hokkaido/1/2022 (H5N1). Results were compared with those obtained using an antigenically homologous vaccine to the challenge strain and a Japanese stockpiled vaccine. Results: All vaccinated juvenile chickens developed sufficient immunity to survive the challenge at 21 days post-vaccination. The rgPR8/VN23 (H5N1) and homologous vaccines markedly reduced virus recovery, suggesting near-sterile protection, whereas low-titer viruses were transiently detected in chickens vaccinated with the stockpiled vaccine. The rgPR8/VN23 (H5N1) vaccine conferred clinical protection in juvenile chickens as early as 8 days post-vaccination. A single dose of the rgPR8/VN23 (H5N1) vaccine provided incomplete protection in laying hens, whereas a double-volume regimen improved protective efficacy. Conclusions: The rgPR8/VN23 (H5N1) vaccine conferred strong immunity to juvenile chickens; however, a refined vaccination strategy may be required to achieve complete protection in laying hens.