Most definitely php. vb.net will be for desktop development. It all depends on what you want your web site to do, for data interaction, php is perfect, just some pages with script, normal html will fine as well.
VB.net can be used to build web applications, but it is fundamentally a client-side application development tool, just using VB code instead of C#, etc. You can also use C# to build web apps, and MS has some nice tools that let you do that with either language. The only issue is that you are limiting yourself to a Microsoft environment, and that is not suitable (in my opinion) in today's world.