You can think of a partial class as code in separate files. Then compiler will merge it in one class file.

One solution is to just make new inherited class and override constructor.

Second solution is to make Shared (static) method builder:

Partial Class MyAwesomeClass
    Public Shared Function Create() As MyAwesomeClass
    ' your code goes here
    ' calling base instance creation


Third solution is to make different signature for second constructor, because you cannot have two methods in class with same name and signature, e.g.

Partial Class MyAwesomeClass
    Public Sub New(Dim x As Integer, Dim buildWithNewAwesomeImplementation as Boolean) ' 
        Me.New(x) ' calling base constructor
        If(buildWithNewAwesomeImplementation)
          Line4_of_code()
          Line5_of_code()
          Line6_of_code()
        End if
    End Sub
End Class


First solution seems more reasonable. 

.NET - VB - OOP - Partial Class - How to create Partial Constructors
I've had a few suggestions as to an "official" name for this tuning, but what do you lot think?

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