Originally Posted by
yycworldtraveler
Visa not required provided continuing on the same day and
holding confirmed return or onward ticket.
Leaving the airport transit area is not allowed.
If arriving by Vietnam Airlines only, it is possible to obtain
a transit visa on arrival, provided holding letter from
Vietnamese Immigration confirming that a visa will be issued
on arrival. Fee: USD 25.-. Also see: For details, click here .
Neither of these provisions apply in the OP's case.
The first does not apply because if he is travelling with luggage (which AirAsia cannot and will not check through the bags to Asiana) he will need to exit the transit area. This rule only applies for passengers whose bags are through tagged and who remain within the international part of the terminal for an airside transfer (for example, someone flying SIN-SGN-ICN on SQ/OZ or someone flying FRA-SGN-SYD on Vietnam Airlines).
The second provision, since it specifically lists Vietnam Airlines, also does not apply to the OP, who will be flying AirAsia and Asiana. This provision is in place to enable connections on VN from cities served only from HAN to cities served only from SGN (an itinerary similar to CDG-HAN-SGN-SYD, let's say). The only way to perform such a journey would be to include a domestic Vietnamese flight, for which the person would need to enter Vietnam - and this is where the VN visa waiver applies.
JettaTurbo, I would recommend that you either get a Vietnamese visa in BKK, or that you fly TG as
yycworldtraveller suggests.