Beaches
Thailand wins on beach infrastructure: more islands, better hotels, easier boats, calmer water in the high season. Vietnam has beautiful beaches but fewer of them are set up for a two-week holiday.
If the trip is mostly beach, Thailand. If it is mostly landscape and cities, Vietnam is at least equal.
Food
Both are exceptional. Thai food is more accessible internationally and slightly sweeter; Vietnamese food is lighter and more herb-driven.
If you want to cook and eat seriously, do both — they are an hour apart by air.
Logistics and ease
Thailand is easier: better roads, more English, more domestic flights, more hotels at every price. Vietnam rewards more effort with fewer crowds and lower prices.
For a first trip in Asia with children or older travellers, Thailand is the lower-risk choice.
