Can I code ISAPI filters / extensions with Visual Basic?

Not directly, because ISAPI filters and extensions require functionality only found in C++ (or Delphi). For more information, see Designing ISAPI Applications at MSDN Online. The reason most people ask about this is that they want to know how to access http://theirsite/theirDLL.dll?whatever much like expedia does (e.g. http://www.expedia.com/pub/agent.dll?qscr=cmfd&itid=&itdx=&itty=&ecid=&subm=0). 
 
However, there are some links out there that allegedly help make this possible (though the overhead of hitting the VB runtime with every request to your web server is something you definitely want to weigh in your testing). You can see some of Microsoft's benchmarks here. 
 
    VB Bridge 
 
    SpyWorks 
 
    OLEISAPI (outdated) 
 
I have not tried any of these methods, so do not vouch for their validity.