There is also one in Hoxton but its different. The one in Soho is better. Cây Tre's got two sisters: kêu! and Viet Grill both in Shoreditch. Cây Tre /Soho stays my favourite.
My starter recommendation will be the Vietnamese Spring Rolls following by a Beef Pho and a Saigon beer as a drink.
I also recommend the Bun Sa (pic below), the Roasted Spring Chicken Royale, the Vietnamese Beef salad (spicy) are very tasty too.
The Pho is a vietnamese noodle soup consisting of broth, linguine-shaped rice noodles called Bánh Phò, a few herbs, and meat.
Bun Sa (hanger steak)
Vermicelli Bowl with veggies and lemongrass sautéed.
For a cheaper variant you've got also The Pho restaurant. Nice! (not as nice than CayTre) but cheaper. They have got several all around London.