Let's first see in how many ways the numbers can be placed with all neighbours coprime.
The four even numbers must be non-adjacent, and the only way to have four non-adjacent vertices of a cube is when they form a regular tetrahedron. There is really only one way to arrange four numbers in a tetrahedron up to rotation and reflection.
Once you have the tetrahedron of 4 even numbers, the 9 cannot be adjacent to the 6, which leaves only the vertex of the cube diametrically opposite the 6. The same goes for the 5 and the 10.
That leaves the 7 and 11, which are coprime to everything. They can be placed either way in the last two spots.
That means that up to rotation and reflection, there are only 2 ways to arrange the numbers. The group of symmetries of the cube has size 48, so after rotations and reflections there are $2\cdot48=96$ valid arrangements.
Now for the probability:
There are $8!=40320$ ways to arrange the numbers, of which $96$ are valid. The probability is therefore $\frac{96}{40320}=\frac{1}{420}$.