evo ti rešenje, nadam se da će ti malo proširiti pogled na programiranje
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
Dim duzStr, br1 As Integer
Dim strMidString As String
br1 = 0
duzStr = Len(TextBox1.Text)
For i = 1 To duzStr
strMidString = Mid(TextBox1.Text, i, 1)
If strMidString = "a" Or strMidString = "e" Then
br1 = br1 + 1
ElseIf strMidString = "i" Or strMidString = "o" Then
br1 = br1 + 1
ElseIf strMidString = "u" Then
br1 = br1 + 1
End If
Next
TextBox2.Text = br1
End Sub
pozz!