C000-CFFFF, however the VDC init routines are E000+ and part of the KERNAL modifying it for things that are not 80x25 is not trivial, if you want to go smaller it should not be that hard and honestly just use the Window Command to get it, if you want to larger in the X or Y you will run into pain, see my post in VDC about 80x50.. Computes mapping the C128 will be invaluable to you.
Yet is it possible to display it correctly in Z64K (Z64K problem emulates not, even emulates incorrectly 100x50 with attributes) - no flickering. Two modes - 80x50 and 100x50. Also on C1901 monitor without flickering by adjusting VSync, but C1084 and others have not available VSync correction, to it flickers - but not characters, flickers whole screen. It needs to adjust VSync to lower value, so also it will be not 50 lines, it needs less lines, but I want to use as many as possible. But without Editor for 100x50 it is for direct mode useless and 80x50 allows use only 25 lines even when is 50 displayed - it is possible use whole screen in both modes only by direct write to VDC-RAM.
Commodore 64 was great, Commodore 128 is bigger, better, faster and more powerful... Commodore 65 was almost here, but C256 is coming and it will be earthquake...