In Data Protector 9.05 there has been a new omnirc variable which controls the maximum number of Virtual Machine Disk Agent objects that are started concurrently.
# OB2_VEAGENT_DISK_CONCURRENCY=<vm disk concurrency>
# Default: 10
# This setting is used to calculate a safe number of disk threads to be started
# in parallel. This is the total number of disks that can be run in parallel at any
# given time. The disks selected are from any of the running VM threads.
# Number of disk threads = VM disk concurrency - 10% (but at least one thread)
# Each of the disk thread creates the connection to the vCenter apart from the runnning
# VM thread. Also take into account that each vSphere Client opens a connection too.
Set this parameter in the omnirc file on the backup host to achieve the desired results.