Overall this trend benefits Vietnam. But the specific effect depends on individual cases.
Take Samsung for example, after shutting down its factories in China Samsung actually began to outsource both the design and the manufacturing of its lower end phones to Chinese ODMs. So it's really a matter of Samsung making itself more asset light and efficient. China now plays a bigger role in Samsung's supply chain as Chinese companies now both design and manufacture Samsung phones.
Samsung's plan to outsource a fifth of its smartphone production to China next year may help it compete with low-cost rivals such as Huawei and Xiaomi but it's a strategy fraught with risks, people with familiar with the move said.
www.reuters.com
Taiwanese OEMs like Pegatron are moving some of their operations to Vietnam but at the same time Apple added Chinese OEM Luxshare to its supply chain. This again moves a Chinese company up the value chain (Luxshare did a lot of outsource works for Foxconn before becoming a direct supplier to Apple).
For tires and auto-parts it's mostly to avoid tariffs though, so likely a net loss for China. But Vietnam has a large population and a growing economy so its domestic demands for automobile will increase. If the trade war ends Chinese manufacturers can shift the focus of new investments back to China (if Chinese factories are more productive) and increasingly use Vietnamese factories to supply Vietnam's domestic market. Really depends on how the trade war goes.