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 actual size of the write operation during a backup job can be verified using Procmon. Filter the output of Procmon to entries from beremote.exe with writefile operations with the details column showing the actual length of the write operation. Additionally the VMWare Agent backups can be debugged.
