My Vietnamese visa will expire on May 7th and my sister has booked me a ticket from Ho chi Minh to Philippines on May 8th, my flight is around 1:45 am. Is this okay?
No. You'll be in Vietnam without a valid visa for one hour and 45 minutes.
Checking in and going through security and whatever control there is on exiting a country 2-3 hours before your flight is quite normal.
So if you do so the 7th, the authorities might not notice that you're actually going to overstay your visa, and if you do so early (like at 20.00) you might increase the chances (those who check your papers might not yet have set their minds to checking if your plane actually leaves before or after midnight). If they don't notice and you behave nicely in the airport, you might not get in touch with anyone else checking your papers and as such you could get away with just hanging around in the airport until your plane leaves. But you would be breaking the law.
And if the authorities notice what you're doing, it comes down to what they feel about it, if they're in a good mood an you play nice with them, they might let you get away with it, they might also see it as an intentional violation of their rules and put you in jail. (Buying a plane ticket for after your visa expires sounds pretty intentional, and they probably don't care that you blame your sister)
In short: It's not allowed, you might get away with it, the risks involved are quite significant, I wouldn't advice anyone to do it. Contact the airline to hear if they can help you exchange the ticket for an earlier one.