It is possible to do a transit in SGN. There is a transit counter at the left end of the immigration hall. They do check tickets and passports there and send you thru a shortcut into the departure hall.

There are two lounges there (one is the VN airlines, the other is a SASCO(?) lounge which is used by most star carriers. For this lounge you need an invitation (which you get at check in). Without invitation (even with a C or F boarding pass or a status card) you cannot get in.

There is one snack bar and a few (overpriced) shops, otherwise fairly boring. Also not sure if the departure hall stays open at night.

If you need a visa check the San Fransisco consulate. They dont need your passport, just a form, pictures and money and they will mail you the visa on a separate piece of paper.

Early in the morning and late at night it takes less than half an hour for a taxi to get into the center of town. Cost is around 5 US$. There is a hotel closer to the airport (Omni? not sure).

Typical time from touch down to hotel is 60 - 90 min. You need to leave around 120 min before your flight - so that does not leave much time to sleep