Dear SBS,
I have cross checked in my system, as while doing STO account assignment "K" is some thing new for me, I don't know much about your business process, why it needs you to do like this, but my system is posting consumption entry& taking GL from GBB-VBR. Please cross check you Account assignment "K" account modifier in your system. Else there is no way that system could pick VAY.
VAY normally used in case of delivery of goods in MTO.
Best Regards
Mohsin Abbasi