There is no equivalent of AF SDK for VB. PI API does not allow you to access AF server. As you mentioned, you can use AF SDK if you are using .NET technology. Is there a reason that you have to use VB in this case?
I guess you can look into to access AF programmatically in VB is through ADO with PI OLEDB Enterprise (you can download the beta version in the Download Center). It is not a released version yet, but you can try it out with the beta version.
VB.NET or VB6 ?