For cities and walking around on your own plus eating/drinking, it is important that you find a central hotel and a city where you are walking or very short drive distance from everything.
For me Bangkok is perfect with regards to this, Singapore is less exotic, but the dining is great and one can go to Chinatown/little India etc. for some interesting on-your-own "light" sightseeing. However, rest of Singapore is very modern and developed. Not yet been to any Vietnam cities, will go in March and can report after this.
From our resent trip to Laos and Cambodia I can, however, highly recommend Amantaka and Amansara. Both very central and possible to explore on your own, as well as daily very good outings to see temples, villages, river/lake etc. Food options outside hotel are plenty, but there is a compulsory charge for half board so we ended up eating in all the time.