I'll be traveling to Bangkok soon and I was wondering if restaurants and vendors accept AmericanExpress?
You'll find that the major restaurant chains (the ones found in Central Malls) accept Amex, as do many of the department stores and major brands. High end restaurants, bars and clubs, as well as 4*+ hotels are also no problem.
Outside of the above then many restaurants accept only visa or master, and even then, they may badger you for cash or a surcharge.
For car rental, Avis and Hertz willingly accept Amex but Sixt will grumble and often refuse. Thai RentACar accept only visa/master. Most PTT, Shell and Caltex gas stations accept Amex.
In general, establishments in keeping with Amex's lifestyle brand will normally accept the card - but then you can enjoy such places anywhere in the world. Bangkok is best enjoyed 'raw'. So to get the most out of your trip, I'd recommend a pocket full of cash and a visa/master as backup.