The fermentation is most likely done or very nearly done. I don't have experience with that specific yeast, but it's very reasonable for a beer to be done fermenting in a week or less. In general, higher temperatures produce faster fermentations.
If it is finished, then it's too late for a diacetyl rest, but I don't think this yeast even needs one. Warmer temperatures help the yeast clear up the diacetyl they produce. It is a lager yeast even though it ferments at higher temperatures than other lager yeasts.
Your beer will definitely benefit from lagering at ~34 deg. F. for a few months. It will clarify and some of the harsher tasting elements (if any) will fade away and you'll be left with a brilliantly clear, smooth, malty Oktoberfest.
In fact, all beers benefit from cold conditioning (lagering), even ones made with ale yeast.
On Deck: Jamil's Vanilla Robust Porter
Fermenting: Orange Blossom Mead
Kegs: Element 56 Pale Ale, Ron's Belgian Blonde, Summer'n Saison, Furloughktoberfest '09, Grateful Pale Ale, Sam Adams Cream Stout Clone, EdWort's Apfelwein