Thank you! All for your time and support
This issue is solved, I made few changes like:
1) Use the same user for the RFC user in ECC and SRM
2) as mentioned in one of my above response to lower down the sys_type value in bbp_backend_dest form ECC6 to ECC5.
3) Global setting in Vendor Sync.
and few more...
Not sure exactly which one worked out, will try to back trace it and keep it posted here if I found that.
Best Regards,
Varun