I don't think anything can edit AVCHD files smoothly at the moment. Vegas lets you see a jerky preview that is OK for basic cuts editing, but it's not much fun compared with DV editing.
If you need to add a lot of effects and filters, it's probably better to convert the footage to an intermediary codec, such as Cineform, Canopus HQ or Lagarith. Takes up a lot of hard disk but it is a lot smoother during editing.