Buy your alcohol at ICN just for ease. i think you can bring in 4lts between you both but do check that. As a side note, not sure what your partners family likes to drink but i've found a bottle of cognac goes a long way with the men in the family over there
As for bringing goods into Vietnam, my wife and i always bring toys and clothes for her family, normally just in an extra suitcase, never ever had a problem (apart from my mother who bought some toys and had her luggage checked in SIN)
Fair warning if you are bringing anything like watches, electronics, cameras etc you can be held to account for taxes payable if bringing them into the country.
If you are bringing any of those just remove them from any packaging so they don't look new and you can claim them as your own thus avoiding taxes.
I see cardboard boxes coming into Vietnam all the time, one thing i would look at is getting it plastic wrapped in SFO which deters baggage handlers from opening and pilfering.
My wife and i will often travel with a cheapish duffle bag which we put clothes into on the way over leaving room for gifts in the suitcases then on the way back the duffle just folds up into a suitcase if it's not needed.