November 2016 Update - Apple has removed support for PPTP in the latest version of iOS. If you are using iOS 10 or above then you cannot use PPTP.
iOS has built-in support for PPTP and L2TP/IPSec. Just follow the instructions in the client area of the VPN.ac website. Remember to use the special China servers which you can find in the "Instructions for users in China" article in the knowledgebase of the client area. Alternatively you can directly enter an IP address instead of using the server addresses. After you connect successfully using PPTP or L2TP/IPSec in the Windows or Mac application and then just copy the IP address and enter it directly as the server in the iOS settings. This will also avoid DNS problems too.
I recommend that you follow my instructions here instead of the Tutorial the VPN.ac website. My solution is easier and doesn't require iTunes or emailing the repository files.
Step 1 - Load this page (www.tipsforchina.com/vpnacios.html) on your iOS device using the Safari browser.
Step 2 - Download the OpenVPN Connect App by OpenVPN Technologies. If you are using the China iTunes App Store then you will not see this app. Follow these instructions first before continuing if you don't have access to the USA App store.
Step 3 - Open this page using iOS Safari browser and then click the link for "repository for users in China"
Step 4 - Download a few of the files from each UDP folder: AES-256-UDP, AES-128-UDP, and BF-128-UDP. The TCP files will not work but you can contact VPN.ac support if you need TCP. During my testing the BF-128-UDP was working the best. I recommend Los Angeles, San Francisco, Seattle, Hong Kong, Japan, and Singapore locations. Touch your preferred files and then choose the option "Open in OpenVPN"
Step 5 - Click the green symbol with a plus sign to import the profile into OpenVPN Connect.
Step 6 - Enter your VPNAC User Name and Password and then connect by touching the toggle switch.