How do I express "if value is not empty" in the VBA language?

Use Not IsEmpty().

For example:

Sub DoStuffIfNotEmpty()
    If Not IsEmpty(ActiveCell.Value) Then
        MsgBox "I'm not empty!"
    End If
End Sub

It depends on what you want to test:

• for a string, you can use If strName = vbNullString or IF strName = "" or Len(strName) = 0 (last one being supposedly faster)
• for an object, you can use If myObject is Nothing
• for a recordset field, you could use If isnull(rs!myField)
• for an Excel cell, you could use If range("B3") = "" or IsEmpty(myRange)

Extended discussion available here (for Access, but most of it works for Excel as well).

Try this:

If Len(vValue & vbNullString) > 0 Then
  ' we have a non-Null and non-empty String value
  doSomething()
Else
  ' We have a Null or empty string value
  doSomethingElse()
End If