I think this a limitation and I cannot see a way to solve it easily. Although LIMIT_MB should work there's a know issue that would be related
In the other hand we have also the fact that the vcenter template has disks that are not visible by vOneCloud. When you instantiate a template a VM is cloned from that template and hence the disks are cloned but still those disks are not visible to vOneCloud. I may be wrong but when the VM is instantiated the scheduler would check if the VM can be deployed in any of the datastores and would use the image size of the disks inside the template to check if total size is higher than the limit, but as those disks are not visible right now the scheduler does not know the full weight of the VM and then it thinks that no limit is being reached. For next vOneCloud, those disks in the templates should be visible and therefore that limit should be controlled better.