Coming back
Return entry into Vietnam
The whole point of the trip is the stamp you collect on the way back into Vietnam. That re-entry is what starts your next stay, so the paperwork before you leave Saigon matters more than anything at the Laos border.
You need a valid Vietnam e-visa before you cross back in. For most people on this route that means a new 90-day e-visa, single entry at 2,400,000 VND or multiple entry at 3,100,000 VND, filed through the immigration portal a few working days before you travel. Standard processing runs 3 to 4 working days, so file it at least a week ahead of your departure date and print the approval. You cannot sort this at the border. The immigration officer at Moc Bai will not accept a phone screenshot and a promise.
On the way out, you exit Vietnam at Moc Bai and get the exit stamp. That stamp, photographed and sent to the visa desk, is what the team uses to trigger the new e-visa processing if you booked the add-on. On the way back, you re-enter Vietnam at the same crossing and present the printed approval. The officer stamps you in, and your fresh 90 days start from that moment.
Timing the return matters. The Moc Bai passenger checkpoint is open around the clock, so you are not racing a closing gate. But you do need the whole two-day arc to fit your schedule: out through Moc Bai, across Cambodia, the Laos stamp, back through Cambodia, and in through Moc Bai again. That return leg is the same 500-plus kilometer drive you did going out, just reversed, so budget a full day for it and do not plan a same-day flight home from Saigon after the run. Overnight in the city, then fly the next day.