User blog comment:Xilinoc/Ch. 541 - THE BLADE AND ME 2/@comment-5606268-20130612163809/@comment-10630156-20130613012534

I think that it was actually massive amounts of reiatsu from his real zanpakuto (hollow ichigo) that prevented his Bankai from being stolen I do remember the vandenreich did say something like the only person who could steal Yamamoto's bankai was Ywach maybe only high class stern ritter can steal bankai's with that much power.

I agree with the training fight with one of those two (or both) and Hollow Ichigo coaching (or abusing) from ichigo's subconscious