Well, to state the obvious, they are both in the 8" HLCDs.

The Skylon seems to have the same clamping system and the pod looks to come out of the same mold, but with a couple tweaks to the mold, but thats probably where the similarities end. The xm9 is now coming with stainless steel clamps. This is a huge step in finish durability. I do not know what the skylons have, but in the pics, they look to be chrome plated.

As noted, both in the 8" HLCD class. I know what the cone dia of the XM9 is, but not the vector-8. There would have to be a fair amount of size diff to equal anything, so move on to the compression driver. The vector is a 1" and xm9 is a 1.5". This give the edge to the xm9 in thermal power handling.

comparing retail to retail, the edge goes to the xm9. I have not personally heard the vector, but I have an idea of how they will sound. If you have an opportunity to demo any audio gear, its worth it. What ones ears like yours may not.