I believe that you are assigned a VAA seniority number when you join VAI, so joining sooner rather than later gets you ahead in that department.
Have any of the 'holdee's' out there managed to have this confirmed by HR either way?
I'm hoping to get an answer early next week. IMHO, if the seniority does count, then this would be a good option long term; i.e. trying to get an upgrade in 6 years time!