Support of NDIS 3.0 was removed from Windows 10 starting 1809, so the NDIS 3.0 of VirtNet can’t be used anymore. You can check this thread for the details and temporary NDIS 6.0 VirtNet driver replacement:
That depends of which installer you have downloaded. Two of them (MSI ones) install driver only (x64 or x86 depending on the platform), the third one contains more demo binaries and includes ndisapi.dll.
The source code for ndisapi caт be found here. You can use as a static or dynamic library (or even .net class library) depending on your requirements.