The DrawingDimension object has a Layer property. You can access DrawingDimension.Layer.Name and test that against the layer name you want to arrange dimensions for. If it matches, add the dimension to a SelectSet then run the arrange dimensions command on the selected dimensions. Here is an example of how this could look.
'Get a reference to the docuemnt and sheet
Dim oDoc As DrawingDocument = ThisApplication.ActiveDocument
Dim oActiveSheet As Sheet = oDoc.ActiveSheet
'Get a reference to the SelectSet and clear it
Dim oSelectSet As SelectSet = oDoc.SelectSet
oSelectSet.Clear
'Loop through the dimensions and select only ones that match the layer(s) we want
For Each oDim As DrawingDimension In oActiveSheet.DrawingDimensions
If oDim.Layer.Name.Contains("Dimensions") Then oSelectSet.Select(oDim)
Next
' Get the CommandManager object.
Dim oCommandMgr As CommandManager
oCommandMgr = ThisApplication.CommandManager
' Get control definition for the arrange dimensions command.
Dim oControlDef As ControlDefinition = oCommandMgr.ControlDefinitions.Item("DrawingArrangeDimensionsCmd")
' Execute the command.
Call oControlDef.Execute
I hope this helps you.
