for your budget and time frame, i thnk you should keep things neat and simple, go BKK of course, then siam reap. all can be done by bus or train, but consumes time. if you wanna go for another locations, have to take a return flight from kl--bkk & bkk--kl to save time.
for me, if for two (assuming couples) then I will do chiang mai if can instead of saigon or anything else mabye.
US$500=RM1900 for 2, then one person budget RM1k, if including visa.. ar har.. you cann't get to siem reap i think, you may need to do train/bus to phuket/koh phiphi then bangkok then fly back to KL. tha's enough for your budget and time i think.