click here for the thread regarding differences in Ong Bak dvds.
there are currently no legit english-subtitled versions. official dvd's with english subtitles were never released so that the theatrical release wouldn't be easily overlooked in favor of a dvd. you really don't need to understand the dialogue in order to get a basic understanding of the movie. if you want to catch the nuances, then i guess you will need subs. i think some of the humor may be lost in translation though.
the vcd does indeed have lower picture quality and darker shading in the cave scenes. i tried watching the vcd on a 20' projection screen i have at the theater i work at, and it was pretty tough to see what was going on. the dvd (thai) was crystal clear; the cave scene was not too dark.
if you really feel like you need the english subtitles, i recommend you simply wait for the official english dvd. i know it's tough... in the meantime,
pick up the vcd just so you can watch the action. worry about the plot later!