http://smileykeith.com/2013/01/08/nstableview-vim-keys/ Override keyDown to use vim keys in something. Like an NSTableView. Only goodVimKeysSandbox.mm works perfectly in the OS X sandbox.

goodVimKeysSandbox.mm

if ([theEvent keyCode] == 38) { // j
    unichar down = NSDownArrowFunctionKey;
    NSString *downString = [NSString stringWithCharacters:&down length:1];
    NSEvent *newEvent =[NSEvent keyEventWithType:NSKeyDown
                                        location:theEvent.locationInWindow
                                   modifierFlags:theEvent.modifierFlags
                                       timestamp:theEvent.timestamp
                                    windowNumber:theEvent.windowNumber
                                         context:nil
                                      characters:downString
                     charactersIgnoringModifiers:downString
                                       isARepeat:theEvent.isARepeat
                                         keyCode:down];
    
    [super keyDown:newEvent];
} else if ([theEvent keyCode] == 40) { // k
    unichar up = NSUpArrowFunctionKey;
    NSString *upString = [NSString stringWithCharacters:&up length:1];
    NSEvent *newEvent =[NSEvent keyEventWithType:NSKeyDown
                                        location:theEvent.locationInWindow
                                   modifierFlags:theEvent.modifierFlags
                                       timestamp:theEvent.timestamp
                                    windowNumber:theEvent.windowNumber
                                         context:nil
                                      characters:upString
                     charactersIgnoringModifiers:upString
                                       isARepeat:theEvent.isARepeat
                                         keyCode:up];
    
    [super keyDown:newEvent];
} else {
    [super keyDown:theEvent];
}

vimKeys.mm

if ([theEvent keyCode] == 38) { // The letter 'j'
    CGEventRef e = CGEventCreateKeyboardEvent(NULL, (CGKeyCode)125, true);
    CGEventPost(kCGSessionEventTap, e);
    CFRelease(e);
} else if ([theEvent keyCode] == 40) { // The letter 'k'
    CGEventRef e = CGEventCreateKeyboardEvent(NULL, (CGKeyCode)126, true);
    CGEventPost(kCGSessionEventTap, e);
    CFRelease(e);
}

zbadVimKeys.mm

NSUInteger flags = [theEvent modifierFlags] & NSDeviceIndependentModifierFlagsMask;
NSNumber *shiftPressed = @(flags & NSShiftKeyMask);

if ([theEvent keyCode] == 38) { // j
    NSUInteger index = [[self selectedRowIndexes] lastIndex] + 1;
    if ([shiftPressed boolValue]) {
        [self selectRowIndexes:[NSIndexSet indexSetWithIndex:index] byExtendingSelection:YES];
    } else {
        [self selectRowIndexes:[NSIndexSet indexSetWithIndex:index] byExtendingSelection:NO];
    }
} else if ([theEvent keyCode] == 40) { // k
    NSUInteger index = [[self selectedRowIndexes] lastIndex] - 1;
    if ([shiftPressed boolValue]) {
        [self selectRowIndexes:[NSIndexSet indexSetWithIndex:index] byExtendingSelection:YES];
    } else {
        [self selectRowIndexes:[NSIndexSet indexSetWithIndex:index] byExtendingSelection:NO];
    }
}