There's not too much vegan food in the Loop, but my friend and I gave Star of Siam a shot one evening. The atmosphere is kind of nice and cozy and the service was fairly prompt and friendly.
That said, the food was terrible. The fried tofu appetizer was chewy; clearly the tofu had been fried hours ago and had been left around to sit under a heat lamp for a while. The egg rolls were okay, but a bit greasy for my likes.
The main dishes were the worst offenders. My pad prik (chili stir fry) looked nice, especially the way they cut the bamboo shoots to look like noodles, but the dish had no flavor. There was lots of oil, but no taste and I had to douse the dish with soy sauce to make it barely worth eating.
Similarly, the pad kraw pao (basil stir fry) was watery and flavorless. They put way too much tomato in the dish (why did they put tomato at all? it's not commonly used in Thai cooking), which sucked out the flavor, and the tofu was barely cooked.
The only plus point: they offer brown rice for an additional $1 or so.
Go try some other Thai restaurant in the Loop, as this place has nothing going for it.