The AREX train now links all the way through from ICN to Seoul Station so journey times are much reduced, although still at 43-53 minutes admitadly it's still a fairly long way compared with many airports. Limosine buses also don't take too long as long as it's not rush hour.

The train is due to be upgraded in years to come so that they get progressively faster and journey times further reduced.

If you want to get to Vietnam, you could try the LH flight that makes a brief stop in BKK before proceeding on (I think to SGN). Not sure if you have to get off during the stop in BKK or not though.

From SGN you'd be pretty much reliant on SQ for your onward journey which would mean a transit in SIN.

Personally though I think RedChilli's suggestion is better if you can find availability.