just wanted to share our experience as it might be relevant for some other people. We are two travellers from the UK.
We made our China visa application with a Visa Agent through Hanoi Hostel as we arrived in the city a Wednesday afternoon, wanted to go to Halong Bay on Friday and Saturday, then leave for China on Sunday. Obviously the embassy is closed at the weekend so we only had 2.5 days to apply, and would only be in Hanoi for 1.5 of those personally.
We paid for the 2-day express Visa service for this reason. It cost us US$85 each (express applications made in person are US$60).
The visa agent collected our passports from the hostel, along with one passport photo, our home address and our mothers' names. The interesting thing is this all we needed. No onward flight details, hotel reservations, bank accounts etc....I guess this might suit people who are happy to pay the extra US$25 dollars so they don't have to be organised and get itineraries planned and hotels booked in advance, which are required if you make the application in person.
The hostel also has a normal speed service which takes 3 working days and costs US$55 (instead of US$30 if made in person)
Hope this is useful!