Importing the namespace isn't necessary. The Imports statement is a sort of shortcut so that you don't have to type the namespace again and again. Although it would work.
You could do something like this:
Dim str As String = Microsoft.VisualBasic.Left("Test", 2) ' str = "Te"
The more .NET way of doing that is to use the SubString Function of the String class. The above code would become:
Dim str2 As String = "Test".Substring(0, 2) ' str2 = "Te"