Despite the alarming news on the subject, using a VPN on an Apple device is not a problem.
However, you should know that once in China, the App Store will prevent you from downloading (or updating) VPN applications. If you want to use a VPN on your Apple device, you will have two options: downloading the app outside of China or configuring it manually (VPN websites tend to have detailed instructions on how to do this).
Remember that once the VPN is connected, you will be able to download or update the app without any problem.