A For Each loop would iterate the controls in the order they were added to the form (although, I don't think that's specified or guaranteed in any way), like it would with any other Collection type.
For Each works by pulling the collection's enumerator. If you look at MSForms.Controls in the Object Browser (F2), right-click a blank area in the object browser and check the "Show hidden members" option, you'll see a hidden _NewEnum member on the type.
_NewEnum also exists in Excel.Workbooks, Excel.Range, VBA.Collection, and everything else you can iterate with a For Each loop: if you write a custom collection class, you'll need to have a NewEnum() As IUnknown member with a special member attribute too (VB_UserMemId = -4) - this enumerator is what the VBA runtime uses to get the next object reference in the collection.
And it knows nothing of a control's TabIndex. The TabIndex determines the order controls get focus when the user hits the Tab key to iterate the controls - it has nothing to do with a For Each loop.
If you need to iterate your controls in a specific order (FWIW I can't think of a legit reason to do that), then you can specify the order in the control names themselves (e.g. txtSomethingSomething1, txtSomethingSomething2, ...), and then retrieve the controls by name, using a For...Next loop (which would be less efficient, since object collections want to be enumerated):
Dim i As Long
Dim currentTextBox As TextBox
For i = 1 To 20
Set currentTextBox = Me.Controls("txtSomethingSomething" & i)
currentTextBox.Value = Application.WorksheetFunction.VLookup(...)
Next
Alternatively, you could store the ColumnN index you need, in each control's Tag property - and get a much more efficient piece of logic (robustness is arguable though.. there are many ways to go about this, that's just one way):
For Each ctrl In Me.Controls
If ctrl.Tag <> vbNullString Then
columnN = ctrl.Tag
ctrl.Value = Application.WorksheetFunction.VLookup(...)
End If
Next
Notice I used Me.Controls instead of PipelineUpdateForm.Controls; I'm assuming this code is living in the form's code-behind - if that's the case, then you should be referring to your form instance with the Me keyword, not the form's name. You'll inevitably run into fairly common issues, sooner or later, if you keep referring to a form's default instance within that form's code-behind. My UserForm1.Show article from last October covers it all in details.
Side note, you probably want to pull Sheets("PipelineRawEntries").Range("B2:AS4") into a local object variable, instead of dereferencing the same object over and over and over, at each iteration of the loop:
Dim lookupRange As Range
Set lookupRange = ActiveWorkbook.Workheets("PipelineRawEntries").Range("B2:AS4")
And then use lookupRange in the VLookup function.
