there are no appointments any more. Everything is done online. We submitted the application on the first Monday after we arrived (we got to Da Nang on Saturday). They did ask for further proof of being in Vietnam. We had submitted our Airbnb receipt, but I guess that wasn't enough by itself, so they asked for the picture of our arrival stamp for Vietnam from our passport and also a picture of our boarding pass. I sent these immediately and still got my E-visa within the 5 business days.
We had no issues with the Ho Chi Minh location for our Non-O. Took 5 business days to process. Perhaps a few extra days if they require additional info, but remember it's working days, so allow for that.
You have to provide "proof of current location" - we had to provide our accomodation receipt, passport stamp for Vietnam, and our boarding pass (make sure you keep your boarding pass!)
Key word "hopefully". So many stories recently of people being denied entry in this scenario. Immigration is cracking down on 'tourists' who are spending too much time. Probably best to get a visa or use a fast track service.