Hi,
I suggest keeping your original plan of spending 5 nights in Berlin. See the other areas of Berlin aside from just Mitte. Historic Berlin is mainly east of Brandenburg Gate. If you want the Vietnamese and Chinese restaurants, you'll find them especially in Wilmerdorf, Charlottenburg, especially on Kantstrasse.
Go out to eastern Berlin areas ie, Berlin Ostbahnhof , Jannowitzbrücke, even more. Customers in these places are exclusively German.
One of my favourite German restaurants with a turn of the century atmosphere is in Koepenick, next to the Rathaus. Take the S-Bahn 3 from Berlin Hbf. From the S-Bahn station Koepenick to the Schloss (chateau) , ie that 40 min walk or so are more German and italian restaurants.
The oldest restaurant in Berlin near Alexanderplatz, (walkable) "Zum letzten Instanz" frequented by locals and some tourists, have been there a couple of times at dinner. Closer to Alexanderplatz is Nikolaiviertel where the very popular restaurant serving Berlin cuisine "Mutter Hoppe" is located, expect that place to be crowded.