Saigon / Ho Chi Minh City is a beautiful city and the streets are filled with great photo opportunities of the local culture, but be careful of thieves on motor-bikes and in markets, especially if you are staying in the city long term.
The motor-bike pick-pockets usually watch foreigners for patterns, so if you are in the city for more than 1 or 2 days please be careful taking your phone out in the streets.
I lived in Saigon for a couple of years and had my phone snatched several times, while making calls during my regular routine.
I would recommend only taking your phone out to take photos, and never walking with your phone or wallet in your hand (always in your bag or pouch). Also, keep it safe while eating, as pick-pockets can take them from table-tops without you noticing.
The average income in Vietnam is only a few hundred per month, so an I-phone represents about 3-6 months of work for the average worker and about an annual salary for street vendors, keep that in mind!