This is one of my favorite Vietnamese places in Montreal and one of the top rated on TripAdvisor. It has a 93% rating on Urban Spoon.
Note that Cafe Saigon is BYO and vegetarian friendly. Also note that they only accept cash.
Here are a few brief reviews from Urbanspoon:
Hidden from view and boasting a cozy, warm interior, Cafe Saigon remains one of Montreal's best kept secrets. The staff are friendly and always remember me when I go in. Even during very busy nights, which happens often, the service is always fast and reliable. The food is scrumptuous and fresh, and you'll be hard up to find a plate that hasn't been emptied. Really, one of my favourite spots. I recommend the Marc special...to die for.
I've eaten there several times and everything I've had was delicious and had a special something compared to other Vietnamese restaurants. If you like spicy food, I recommend the soup satay. Also, the chicken fried rice is really worth trying.
I freakin' love this place. I have a gluten-intolerance, and they are super accomodating. I love family-run businesses where they make you feel like a member of that family. Best thai/vietnamese in the city, BYOB, reasonable prices and stellar service. Try it - but don't tell too many people! I love that it's never super busy and I've never waited long for my food!
We have been going to Cafe Saigon for over 20 years - hands down it's our favorite restaurant. Spring rolls, Sate soup and basically everything on their menu is great. Minh and his family have a wonderful family run restaurant (and now our children bring their friends there also...)