Retrieving a lost Mobiphone SIM card might be one of the easiest thing to do when you are in Vietnam. I recently lost my SIM card, amd I was scared it might take weeks or months before I could get it back, but fortunately it took me less than 30mins to get it up and running again.
If you lost your mobiphone SIM card, Don't panic. Find a mobiphone shop nearest to your apartment. They do have all the Mobiphone shop around, so just call up their customer care line on 9393, since it's in English you won't have much of a hassle. (If you are always lucky like me, you will probably get a nice lady talk to you..she is calm and always ready to help 
). Also note that the English customer care is from 7am to 8pm...duuhhh...Call them to give you the nearest customer care shop available from your apartment...
If you are going to the customer care shop, make a photocopy of your passport if you are a foreigner or ID Card if you are Vietnamese, preferably a colored photocopy so no need to keep checking your small head on the passport to the big one you have after eating way too much Pho 
.
Also try to remember the 5 numbers you have called in the last 3 month, including international numbers if you call any. If you keep shouting and ranting if you don't have these numbers, it is a waste of time. If you don't have the numbers, ask friends on Facebook to help you if you call them in the 3 months timeframe.
In no time, your new SIM card will be handed over to you, but you also need to pay a sum of 25,000 vnd which is like a dollar and some bla bla bla.. Lolxxxx
Don't you just love Mobiphone??? I love Mobiphone, am sure the other telecoms company do the same, but i just tried Mobiphone and its was nice experience to share.
Cheers