Just to confirm that the install method of 1.7a over 1.78 worked for me.
Starting from 5.5U1:
/tmp # esxcli software vib install -d /tmp/BCM-NetXtremeII-4.0-offline_bundle-1796156.zip
Installation Result
Message: The update completed successfully, but the system needs to be rebooted for the changes to be effective.
Reboot Required: true
VIBs Installed: Broadcom_bootbank_misc-cnic-register_1.7a.02.v55.1-1OEM.550.0.0.1331820, Broadcom_bootbank_net-bnx2_2.2.5d.v55.2-1OEM.550.0.0.1331820, Broadcom_bootbank_net-bnx2x_1.7a.10.v55.1-1OEM.550.0.0.1331820, Broadcom_bootbank_net-cnic_1.7a.05.v55.3-1OEM.550.0.0.1331820, Broadcom_bootbank_scsi-bnx2fc_1.7a.08.v55.1-1OEM.550.0.0.1331820, Broadcom_bootbank_scsi-bnx2i_2.7a.03.v55.2-1OEM.550.0.0.1331820
VIBs Removed : Broadcom_bootbank_misc-cnic-register_1.78.75.v55.1-1OEM.550.0.0.1331820, Broadcom_bootbank_net-bnx2_2.2.4f.v55.3-1OEM.550.0.0.1331820, Broadcom_bootbank_net-bnx2x_1.78.80.v55.3-1OEM.550.0.0.1331820, Broadcom_bootbank_net-cnic_1.78.76.v55.7-1OEM.550.0.0.1331820, Broadcom_bootbank_scsi-bnx2fc_1.78.76.v55.1-1OEM.550.0.0.1331820, Broadcom_bootbank_scsi-bnx2i_2.78.76.v55.1-1OEM.550.0.0.1331820
VIBs Skipped:
Then everything goes smoothly to 5.5U1A