I think it's likely we're headed to Laos or Cambodia!
1. Vietnam is unlikely to have hosted a leg; Leo/Jamal are probably on transit in Hanoi.
@JessicaCodyBB19 reposted the photo of Leo/Jamal in Hanoi at 12:26am on 14 Jun, meaning it was taken no later than sunset on 13 Jun, at 6:30pm.
For Leo/Jamal to have completed a leg in Vietnam by then, they would have had to:
(i) arrive in Tokyo at 3:55pm on 11 Jun, taking 1h+ to get downtown, and checking in to the Pit Stop earliest at around 7pm;
(ii) have a ~12h Pit Stop in Tokyo, flying to Hanoi on 12 Jun, taking 40min to get downtown, and checking in to the Pit Stop earliest at around 4pm;
(iii) have a ~24h Pit Stop in Hanoi, and leaving on 13 Jun to be sighted at Noi Bai Airport.
This is an incredibly rushed schedule. In recent seasons, the first Pit Stops lasted ~21h (TAR27, Rio de Janeiro), ~29h (TAR28, Mexico City), ~34h (TAR29, Panama City), and ~17h (TAR30, Reykjavík), and there has always been a calendar day's gap between Legs 1 and 2.
2. There are very few destinations where Hanoi is a logical connection stop.
There are flights from Hanoi to 10 other Southeast Asian destinations. For most destinations, there are multiple better itineraries from Tokyo, so we can discount Bangkok, Chiang Mai, Singapore, Kuala Lumpur, Yangon, Manila, and Phnom Penh. It's also ridiculously unlikely Hanoi would be a logical connection in a two-stop layover scenario.
3. That leaves us with: Vientiane, Luang Prabang, and Siem Reap as possible candidates.
The following work for a sighting on both 12 Jun or 13 Jun:
VN311 from Tokyo Narita to Hanoi, 1000-1330, then:
VN837 from Hanoi to Siem Reap, 1525-1710;
QV312 from Hanoi to Vientiane, 1640-1750; or
QV314 from Hanoi to Luang Prabang, 1910-2030.
