// Get the document's preview settings
DocumentPreviewSettings settings = document.GetDocumentPreviewSettings();

// Find a candidate 3D view
FilteredElementCollector collector = new FilteredElementCollector(document);
collector.OfClass(typeof(View3D));

Func<View3D, bool> isValidForPreview = v => settings.IsViewIdValidForPreview(v.Id);

View3D viewForPreview = collector.OfType<View3D>().First<View3D>(isValidForPreview);

// Set the preview settings
using (Transaction setTransaction = new Transaction(document, "Set preview view id"))
{
   setTransaction.Start();
   settings.PreviewViewId = viewForPreview.Id;
   setTransaction.Commit();
}

// Save the document
document.Save();

' Get the document's preview settings
Dim settings As DocumentPreviewSettings = document.GetDocumentPreviewSettings()

' Find a candidate 3D view
Dim collector As New FilteredElementCollector(document)
collector.OfClass(GetType(View3D))

Dim isValidForPreview As Func(Of View3D, Boolean) = Function(v) settings.IsViewIdValidForPreview(v.Id)

Dim viewForPreview As View3D = collector.OfType(Of View3D)().First(isValidForPreview)

' Set the preview settings
Using setTransaction As New Transaction(document, "Set preview view id")
    setTransaction.Start()
    settings.PreviewViewId = viewForPreview.Id
    setTransaction.Commit()
End Using

' Save the document
document.Save()