I've had a couple of reservations where the VX agent apparently doesn't save my MP number correctly and it just reverts back to my Elevate number. I now just wait until I'm about to check-in to do the change do that I can make sure that I see my MP number as I'm checking in. I figured that if my MP number is on the boarding pass, Alaska would give me credit if anything didn't register properly. So far I haven't had any issues with miles from my VX flights crediting properly to my MP account.