If you are looking for a vegetarian restaurant near the Rama VIII bridge in Bangkok, May Kaidee 2 is a good option. They are located in Samsen Road, between Soi 1 and the canal (see location map), or you can give them a call if you have trouble finding the place at 08-9137-3173. (Replace the 0 in front with +66 if you call from outside of Thailand).
May Kaidee is a vegetarian & vegan restaurant and cooking school with three branches in Bangkok, and they have been around for many years delivering consistently good quality and a menu with lots of choices. If you think that vegetarian food is boring, this place will prove you wrong.
If you are willing to venture a little farther away from Rama VIII, there is another very good vegetarian/vegan restaurant in Thanon Tanao (nearly Khao San Road). They serve international vegetarian food and are called Ethos, you can visit their website here. or give them a call at 02 282 2748.