Vans are a convenient, low cost,comfortable alternative to buses. There are two types of vans available:
The first is vans that follow some of the bus lines. Some are white and purple, some are silver blue. The cost is 15 baht per ride. although there are dozens all over the city, most tourists need to be only concerned with two. The white and purple one that starts at Victory Monument and follows Ratchapralop over to Siam. The other follows the same route but continues through the Ratchaprasong intersection and turns down Silom.
These is an alternative to taking medium haul buses. For less than 200 baht, you can go to destinations like Pattaya, Hua Hin, and Kachanaburi. Most places that require you to go to the southern Bus Terminal are best traveled by van. From central Bangkok it will take about 150 baht by taxi to get to the terminal then more to buy a ticket. Wouldn't it be better to go directly to you destination from a central location easily accessed by BTS? These vans all load up at Victory Monument. Look out fore van drivers calling out their destinations. They leave when the van is full.
Asians are sensitive to the way you get their attention. Pointing with your feet or index finger is considered rude. Hailing a van is no different. If you want to hail a van, do it the way Bangkokians do it--outstretch your arm, palm down, and raise and lower it.