Vias between BGA pads
I ran across a question posted on the Xilinx forum recently about whether vias in BGA pads needed to be capped, as suggested in my post about that subject. We do recommend that the vias be filled and capped with copper in BGA pads. There really isn't enough land surface to not plug the via holes. Some people are okay with an unfilled microvia that's only one layer deep, but even that can lead to excessive voiding.
If you've got a good board house, you may be just fine with vias between to pads down to a BGA ball pitch of 0.5mm (20 mil). Just make sure that there is solder mask between the land pad and the via as is noted in the illustration below.
Consider that two wrongs never make a right... but that three do.