US SIM for data or China SIM for Data and WiFi with VPN
You don't need to have VPN to access Google, YouTube, FB, etc... if you use an US SIM for data with a package from your US service provider. You would have to pay for roaming from your US carrier. T-Mobile US do give you 2G data and texting at no extra charge. 2G data is not fast, it's slow, but it's enough to search, get directions, and email. Just need patient. Once you connected to WiFi, then you'll need VPN to access those sites. If you don't need to access those sites, you don't need VPN.
Then some sites are blocked in China. A VPN will get you get around it when you use the wifi or a Chinese SIM. With you US SIM there is no need as the traffic goes through the US servers.
If planning on using a lot of WiFi or have a China SIM and can't get by without the blocked sites (Google, Google Maps, etc.) then get a VPN.
As for a China SIM, it depends on factors such as what is the cost of an international plan for your phone's data provider or the cost for roaming; and are you planning on using a lot of data. If you're on your phone using data when WiFi isn't available for WeChat (or other favorite social app) then a China SIM would likely make the most sense.
If only occasionally using data, then maybe go with a provider plan or just roaming.
The length of time you'll be in China can affect your choice.