I hope you can get LDAP authentication to work from VB.NET, as this is certainly an important protocol and we plan to use it ourselves for certain things. Unfortunately, we have not started that project and I have no advice to offer at this time.
I thought you should know, meanwhile, that our company supplies a possible alternative to Domino name/password authentication from .NET. Proposion N2N is an data connector for accessing Notes/Domino based on ADO.NET interfaces. N2N includes the ability to verify a Domino internet name/password from .NET code (such as your ASP.NET custom login page) and optionally "run as" that user when accessing Notes data (automatically enforcing the ACL for that user). See
http://www.proposion.com.