Question on the vApp template - is the used disk space 300GB; or the max possible 300GB?
Storage quota is not based on space really used; but on the space that could be used is the all VM's are full.
This at the surface does not sound like the issue - but I just want to rule it out as I know lots of people run into this.
(Example: vApp with 5 VM's - only using 100GB on disk - however each has 3 100GB disks attached. This would be counted as 5*100*3 = 1500GB)