Tang Shaoyang had his stats increased by ten percent, excluding Divine Power which already maxed out from the beginning. He prioritized the Divine Power since all his skills relied on the Heavenly Energy. It was not like it mattered which he should max out first with his Attribute Shift skill.
The attribute bonus was just the bonus from the promise, what mattered to him was that Vandir could help him fight the God Rank. Especially if The God Rank brought another army with them. Vandir told him that he was Lich Emperor, and had control of thirty-two Lichs. These thirty-two Lichs had control over many skeletons, and he had seen one of them have a skeletal dragon. Vandir himself was an army. It was worth using one of the lives of Seven Souls to form a contract with Vandir. On top of that, he had the Lich Emperor join the Tang Empire after Vandir was resurrected. He did not regret making the contract despite being scolded by the other spirits.