Update:
Looks like the limit is in the payload size. We are trying to send payloads of size 8960:
17:22:24.177610 IP6 (hlim 64, next-header TCP (6) payload length: 8960) fe80::250:56ff:fe36:a9a6.16012 > fe80::250:56ff:fe36:a9c9.54600: Flags [.], cksum 0x2285 (incorrect -> 0x79d4), seq 716:9644, ack 2314, win 44, options [nop,nop,TS val 2150401 ecr 279311], length 8928
However, in a simple ping-pong client, server python script, I was able to verify the limit to be around 8878.
Question: is this limit a limitation of vCloud air? Is it configurable?
I am attaching my ping-pong script for reference. You might need to update the IPv6 scope ID from the hardcoded '3' to use the script to repro.