What exactly is your question? Note that you aren't copying every element. for (i = 11; i>0; i--)
This is not reaching i == 0, the first element of the vectors. And i == 11 is not a valid index. Array/vector indices start at 0 and go to size-1.
You could do something like this
for (i = 11; i>0; i--)
d[i-1] = b[i-1];
Please use code tags. That means wrapping the code parts of your posts with [code] and [/code]