He later served in various regional postings and senior offices at the Tang court in Chang'an, and though his conduct met with approbation, in the later stages of his career he was considered controllable.
He later served as a guard commander, and then the officer in charge of storage at Henan Municipality (河南府, encompassing the eastern capital Luoyang).
Once the mourning period was over, he became the secretary general of the supreme court (大理寺丞, Dali Sicheng), and inherited his father's title of Duke of Pengcheng.
At one point, he was accused of an offense lost in history and was demoted to be the military advisor to the prefect of Fang Prefecture (坊州, in modern Yan'an, Shaanxi).
In 750, he was made the deputy minister of civil service affairs (吏部侍郎, Libu Shilang) and given the honorific title Yinqing Guanglu Daifu (銀青光祿大夫).
It was said that Wei was kind and forgiving, and that he was fair in selecting officials while serving as the deputy minister of civil service affairs.
In fall 754, Emperor Xuanzong thus made Wei the minister of defense (武部尚書, Wubu Shangshu) with the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事), making him a chancellor de facto.
In 756, after An's forward forces defeated and captured the Tang general Geshu Han at Tong Pass, Emperor Xuanzong, under Yang's advice, decided to abandon Chang'an and flee to Jiannan Circuit (劍南, headquartered in modern Chengdu, Sichuan).
On July 13,[4] the imperial train, without announcing to the people of Chang'an, left Chang'an, with Emperor Xuanzong attended to by the imperial clan members, Yang, Wei Jiansu, the official Wei Fangjin (魏方進), the general Chen Xuanli (陳玄禮), and eunuchs and ladies in waiting close to the emperor.
Emperor Xuanzong had Li Mao (李瑁) the Prince of Shou visit Wei Jiansu and give him medication.
In fall 756, after Emperor Xuanzong reached Baxi Commandery (巴西, roughly modern Mianyang, Sichuan), he made Wei, in addition to being the minister of defense, Zuo Xiang (左相) -- the head of the examination bureau and a post considered one for a chancellor.
When they reached Shu Commandery, Emperor Xuanzong further created him the Duke of Bin and gave him the honorific title Jinzi Guanglu Daifu (金紫光祿大夫).
In spring 757, Wei was made Zuo Puye (左僕射), one of the heads of the executive bureau, and removed from his chancellor post.