Solution has been found.
json payload should be
{"deployment-name": <put deployment name here as string>, "vm-name": <put vm name here as string>}
works on NFVIS 4.7.x, but doesnt work on 3.12
Please note that API call doesn't return VNC port number as I think it should, but you can connect to port 6001 at first call. With every subsequent call port number is incrementing by 1(6001, 6002 etc)
VNC URL is https://<ip>:<port>/vnc_auto.html