When DD was a bit younger she vomited in situations like that but I agree, it was clearly a sign of extreme distress/stress and something she had no control over let alone manipulation. She is only two now so she would have been too young for that system/theory anyway but even at 18-22 months I've had people suggest it is a diversion tactic or similar. Even now at two or older were she to do that I would say it is a distress signal not one of defiance. They are still so little! They panic, they get overwhelmed, they cannot control their emotions or how their emotions affect their bodies at that level of stress. It's not the same level of stress were you say to give them dinner in a colour they dislike and they throw it everywhere shouting 'no!' and having a tanty. I think at that age they are starting to explore emotions and responses for sure but there is a massive difference between testing boundaries, exploring human emotional response and actually being in an overwhelming or stressful situation (whatever that might be for them personally, at that stage of their development) in which they feel out of control and really distressed.

Hmm long post, sorry! Just made me feel annoyed and protective of our little ones.