The size of the read and write buffers used for the VWware API (VDDK) by Backup Exec can be tuned using the following registry keys:
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Backup Exec For Windows\Backup Exec\Engine\VMware Agent
"Enable Buffered Reads"=dword:00000001
"Number of Read Buffers"=dword:00000004
"Size of Read Buffers"=dword:00000400
"Enable Buffered Writes"=dword:00000001
"Number of Write Buffers"=dword:00000004
"Size of Write Buffers"=dword:00000400
More info.
If you are looking at tuning performance of your backups generally, recommend you to go through the Backup Exec 2012 performance tuning guide:
The...