Amino acids, being chemicals with a particular arrangement of atoms, are completely untraceable if they are pure.
As such, they could come from full organic synthesis from ethane feedstock, or from cultured microorganisms, or from plant protein, or from animal protein. They could by produced as a by-product of some other industry. If the company producing the synthetic meat is not particularly cautious with respect to sources, the vegan pedigree of its product could be questioned.
From the HN conversations about Soylent and Soylent-like products, I have been made aware of religiously-motivated vegans that even feel vaguely guilty about having to kill plants for their food. Those and the vegans motivated by supposed health benefits would not give a fig about the "meatiness" of any synthetic meat, caring more about its nutritional qualities. Those motivated by ethics would likely only care insofar as the existence of a full-synthetic meat could help encourage others to give up animal-harvested meats. If you don't proselytize veganism, you wouldn't care (nor should you).
From my own shallow study of nutrition, the height of my bar for going vegan is the chicken egg. If anyone wants me to stop eating animal products, they must necessarily produce something of equal nutritional convenience to a chicken egg, at a lower price. By my "holy grail" statement, I meat that specifically for proselytizing ethical vegans (i.e. PETA members), as an "Aha! With this, the meat-eaters can no longer argue in favor of their destructive lifestyles based on nutrition or culinary experience!"
