You have to run the GUI for the Vice x64 core itself (separate from the Retroarch GUI) by pressing F10 once you've launched a C64 disk image. Then you get a very rudimentary GUI where you can change it to NTSC and select the Sony NTSC color palette. However, there's no way to save these settings. But it did look noticeably brighter and more colorful than PAL. One thing I vastly prefer about the Vice x64 libretto core is you can use any of Retroarch's shaders with it. CRT-easymode looks magnificent with the C64 (much better than the scan line filter built into WinVice) but the inability to save your settings in the Vice x64 GUI and the requirement of a flipfile to change disks makes it much less preferable. However, if the author decides to update the Vice core and add these features it could be the way to go for c64 emulation.