How do you get the number that is in the center of numbers entered in a textbox vb.net

Since you want to take from the string 2 chars when the string length is even and 1 char when it's odd, you just need to care about the remainder of the division by 2.

Using Integer division (3 \ 2 = 1):
If String.Length Mod 2 = 0, take 2 chars, starting from String.Length \ 2 - 1
If String.Length Mod 2 = 1, take 1 chars, starting from String.Length \ 2

You may want to add a null check and just return the original string if the string length is less than 3 (when the string is "1" or "12", return the string as it is).
Your code could be:

Dim result as string = GetCenterValue(TextBox1.Text)

Public Function GetCenterValue(content As String) As String
    If String.IsNullOrEmpty(content) Then Return String.Empty
    Dim sLength As Integer = content.Length
    If sLength < 3 Then Return content
    Return content.Substring(sLength \ 2 - (1 - sLength Mod 2), 2 - sLength Mod 2)
End Function