Can you try making it an array of integers? Try setting  as the value.
I have had success using an array of integers i.e. json.app.data.amazon_securityGroups = [805, 807]; However, I can't figure out how to determine these identifiers that vRA uses internally. I can query AWS and get the list of Security Groups no problem. However, I don't know how to either translate those values to something vRA will recognize or better for me, pass in the value as the AWS identifier sg-abc12345 and get vRA to understand and apply the correct security group.