Vertigo may not be the cheapest bar in Bangkok, but it is very high up. It is on our list of must-see places when friends come to visit. It affords excellent views of the city and is close to the bars and restaurants of Silom (we've not eaten there, looks too expensive for that).
You'll want to turn up before the sun goes down so you can get your bearings and some good seats. You'll be able to see the river, port, palaces and wats and then watch as the lights come on as the sun sets.
They have a strict dress code so you should dress nicely. Men need to wear closed shoes and long trousers: no beachwear!