Something along these lines will work:

Code:
Private Sub btnCalculate1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCalculate1.Click

Dim valid As Boolean = True

If Not EntryIsValid(Me.txtNumber1.Text) Then
       valid = valid And False
End If
If Not EntryIsValid(Me.txtNumber2.Text) Then
       valid = valid And False
End If

If valid then
'do what needs to be done if the entries are valid
Else
'do what needs to be done if the entries were not numbers
End If

End Sub

Private Function EntryIsValid(ByVal value As String) As Boolean
        Dim result As Single = 0
        If Single.TryParse(value, result) Then
            Return True
        End If
        Return False
End Function
Alterantively, since we aren't using the variable 'result' you can use this:
Although, the intermediate code will make up a variable for you.

Code:
Private Function EntryIsValid(ByVal value As String) As Boolean
        If Single.TryParse(value, Nothing) Then
            Return True
        End If
        Return False
End Function