Hi Guji!
Is there a firewall or NAT in between the VCS and the TMS?
There shall be no NAT and the VCS needs to access the website of the TMS.
Which versions of TMS and VCS do you use? Some combinations forced the VCS to use https
which was often not enabled on the TMS.
As you use HTTP, a tcpdump on the VCS might be helpful.
You could login as root as ssh and execute:
tcdpump -vAnls0 -i eth0 host 10.10.10.20 and port (161 or 80)
and save the external manager settings again and see what you see.