You'll need to look at the RasGetProjectionInfo function and use the HRASCONN pointer that you retrieved earlier. RASP_PppIp should be rasprojection value passed in to get the RASPPPIP struct which contains your IP address. I'd post some C++ code for you to use, but that's not my best language and don't want to embarass myself.
Here's some helpful links to assist you:
RasGetProjectionInfo: http://msdn.microsoft.com/en-us/library/aa377548(v=vs.85).aspx
RASPPPIP: http://msdn.microsoft.com/en-us/library/aa377634(v=vs.85).aspx
Since it appears part of your application is using C# you may want to consider using the DotRas project on CodePlex. It's a C# based wrapper around the entire RAS API. To get the PPP information from DotRas you would need to:
using DotRas;
var conn = RasConnection.GetActiveConnections().Where(c => c.EntryName == "Your Entry").FirstOrDefault();
RasPppIp ipInfo = conn.GetProjectionInfo(RasProjectionType.IP);
From here you can access the ipInfo.IPAddress property to get at the information you want.
Here's the link to DotRas: https://github.com/winnster/DotRas
Hope that helps!