Hi,
Here is some useful info for you,

"A TRANSIT VISA CAN BE
OBTAINED ON ARRIVAL, PROVIDED:
HOLDING A LETTER FROM VIETNAMESE IMMIGRATION CONFIRMING
THAT
VISA WILL BE ISSUED ON ARRIVAL* AND
PASSENGER IS CONTINUING JOURNEY OUT OF VIETNAM UPON ARRIVAL
AT NEXT DESTINATION IN VIETNAM

I strongly recommend that you consult with the nearest Vietnamese Embassy on your problem to verify if you could do the things you mentioned earlier.
( Don't think telling them about the sneaking in part would be a good idea)
It will save you the hassle of being stuck in a difficult and sticky situation.
Also I am not too sure if the Air Asia counter will accept you before you fly to HAN without a Visa knowing you will need to collect your luggage and therefore will need to clear immigration for that purpose.
And I do not think they will transfer your bags from a LC carrier that does not have any baggage agreements with any of the major international carriers, if you decide to remain in transit in HAN.

Have you purchased your tickets to ICN from HAN?
I would recommend flying to countries like Malaysia (KUL) or Singapore on either AirAsia or Thai Air Asia where you don't need a visa and make yourself to ICN from there to connect to your flight to NRT and onwards.

All the same wish you a safe journey!

Cheers!