A word of advice about backups and file copy in general.
DISK is the single most important element of backup, not network, not CPU not memory. DISK.
SELDOM if ever do backup devices saturate the network. Almost never, and I would venture to say CAN'T due to the way they compress / catalog during the backup process. An application isn't capable of flooding the protocol with enough data, considering the destination IO is crazy high. MOST disks cannot exceed the bandwidth (or even come close to maching) network / Fiber speeds. You need multiple streams of backup to allow you to fully utilize the NIC, but your disks most likely (because most people use cheap storage like SATA) won't be able to do much IO anyway.
Disks themselves are basically slow, even SSDD drives, still rely on the application layer for the transfer, and that's where the slowdown occurs.
2 iSCSI NIC's great for redundancy, won't help you in throughput. So don't configure this if you intend on getting speed for this purpose.
I want to shorten the backup time by improving network performance (Tape is not running on max speed)
This won't improve unless your VM's are able to get better disks, but then you are limited to throughput by the application itself. If Tape isn't running at max, its because of the source, there really isn't much you can do with VM Ware, as some of this is API related.
)