Always have their mango sticky rice for dessert, always! (This should be a rule when you go to any Thai restaurant anyway :P)
Truly Thai is a great little spot in the corner of the plaza right beside Vaughan mills. I've been here twice and both times the food tasted great. Both visits I ordered vegetable green curry (it's vegan friendly!) and it's very delicious. The curry is creamy and flavourful with the right amount of spice. If you like extra spice, ask them for some of there hot sauce- that stuff made my lips feel like they were on fire- I caution you to use it sparingly.
The interior has many colourful posters of Thailand that add a nice touch. Waitresses are always friendly and are very knowledgable of the ingredients in the dishes (they said they don't use any dairy in their dishes- how amazing is that?!). Overall, I recommend this place for a decent Thai meal.