Contact lens

Not aware of any chain optometry stores in Saigon.
But there are small ones everywhere, probably all have contacts.
Look for stores, or search a map for this: Mắt Kính
In tourist areas they will probably speak English: in Saigon look around streets Nguyen Hue and  Đồng Khởi in the downtown Bitexco tower area.
But you can save money is less posh areas like if you head down Tran Hung Dao towards D5 Chinatown.

Look up Bệnh viện Mắt Thành phố Hồ Chí Minh in Google maps.  This is the big government eye hospital.  I went there once with my wife but was not shopping for contacts.  Nonetheless if I were, I would think any affiliated optical shop would be up to international standards.

I went to have a check on my previous retinal operation.  I was suitably impressed with the American educated retinal specialist who saw me.  There is a two track foreigner/local system.  You pay more than the locals but your wait in line is way less.  My wife had a regular eye exam and i ended up waiting a while for her.