Netbackup tuning guide, the size of shares data buffers UNIX

According the the Netbackup tuning guide, the size of shares data buffers UNIX should follow the following guideline:

64K for tapes
256K for disk

In a case where we have a VTL. I assume that the virtual tapes are considered as normal tapes for netbackup. So SIZE_DATA_BUFFERS is the value taken for the virtual tapes instead of SIZE_DATA_BUFFERS_DISK. But should virtual tapes not be treated as disks?

And if I consider to treat it as disk, the value I have to put would need to be 256K in the SIZE_DATA_BUFFERS. And then what if I need to set the value for real physical tapes?

Thank you in advance for considering my question.