Hi Rahul, Depends what you want from the experience. I'd go for Bangkok, as Singapore and Hong Kong are quite similar. ... Bangkok is cheaper, Singapore is more expensive. Singapore is more built up than Bangkok, but some would say you get more culture in Bangkok.Thailand is cheaper by far, and KL is a pricey city for SEAsia. A side from the cost of beer I'd say Thailand is more expensive. Little things like the cost of t-shirts and meal in areas like Phuket are bloody expensive compared to anywhere in Malaysia. Taxi's in Bangkok are more expensive (but better service) then KL
Singapore , Malaysia and Thailand is a good option , Vietnam is also a good option , Vietnam has much more to offer , if you planning for a combine trip , I guess it's not a great option, Thailand it self can take more than 15 days and same in the case of Malaysia. you can cover more places if you plan Singapore and Vietnam together.