Vientiane only reject people with the writing that mark "APPLY VISA IN HOME COUNTRY". So far we have seen 4-5 different writing, but not everyone got rejected. Savannakhet will always ask for bank statement.
No problems. At check in counter in USA the airline might ask for your return ticket, other than that you will be OK. I am not familiar with Vietnam visa, but for Cambodia you will be fine( visa on arrival USD30.-).
again, it is always in the hands of the IO how they judge. You did nothing wrong on your part, the IO might feel that you do it not correct (in her eyes). Better to avoid that border and might follow her advise n use air for OUT/IN. Or another border near you.