この場合ならこっちのほうがよさげ？

Public MustInherit Class chara2ch
Public MustOverride Sub itteyoshi()
End Class

Public Class giko
Inherits chara2ch
Public Overrides Sub itteyoshi()
MessageBox.Show("逝ってよし")
End Sub
End Class

Public Class siraneeyo
Inherits chara2ch
Public Overrides Sub itteyoshi()
MessageBox.Show("逝っていいんじゃねーの")
End Sub
End Class

Public Class mona
Inherits chara2ch
Public Overrides Sub itteyoshi()
MessageBox.Show("逝っていいモナー")
End Sub
End Class