Your answers to (i) seem right. As for how many marks you'll get: I don't know, I'm not an examiner. I can't see what working you COULD show, really. I know I wouldn't. That's just my opinion though.
In maths, there's a fun thing called the scalar product (the "dot product" they call it at school).
It's defined like this:
a DOT b = |a| * |b| * cos(angle)
where a and b are vectors, |a| means the magnitude of a, and angle is the angle between the two of them.
So you can rearrange that to get
cos(angle) = [ a DOT b ] / [ |a| * |b| ]
So now all you need to know is what a DOT b is:
I'll show that with an example:
(1, 2, 3) DOT (4, 5, 6) = 1 x 4 + 2 x 5 + 3 x 6 = 4 + 10 + 18 = 32
All you do is multiply the two "firsts" (x-components) together, then the two seconds, then the thirds and add them all up.
Work that out, work out the two magnitudes, substitute it all in and do an inverse cosine and you're away.