When using VB, it is probably easier to use the .NET Connector (NCo) instead of the NW RFC Library directly. Have you taken a look at that already? (Download available at https://support.sap.com/nco )
Assuming that the correct SNC solution & certificate is already installed on the Windows machine (e.g. for use with standard SAP GUI connections), SSO via RFC is pretty simple: you just need to specify a few SNC parameter in the set of logon parameters, like
SNC_MODE=1
SNC_LIB
SNC_QOP=3
SNC_PARTNERNAME
and when using Kerberos: the SNC_MYNAME (when using sapcryptolib, this parameter can be omitted, because the cryptolib automatically knows its own name, if properly installed).
