I had a decent moderate-priced vegetarian curry with rice there and a cranberry juice.
There is not much to say about Siam Thai Restaurant, since it is a run-of-the-mill Thai restaurant. As far as I am concerned there is no justification for the label "vegetarian-friendly" at least not more than any other random Thai restaurant.
That said there was nothing wrong with Siam Thai in particular. The juice however is from concentrate and with lots of sugar, so be warned.
At very least they knew the term 'vegan' and could recommend from the menu. (If you are vegan, ask before ordering, as some dishes are not vegan although no not-vegan ingredient is displayed.)
Cons: no vegan labels