Is this difference expected?  I'm not familiar with the Lao script, so I
don't know if one of the displays is wrong, though this quote from
https://en.wikipedia.org/wiki/Lao_alphabet seems to suggest that either
way is possible: "Vowels can be written above, below, in front of, or
behind consonants, with some vowel combinations written before, over and
after."  This is m17n-lib-1.7.0 and libotf-0.9.13 from
http://www.nongnu.org/m17n/, which are AFAIK the latest released
versions (there's a patch for Bengali in etc/PROBLEMS, but I don't know
of any problem report about Lao).  (Regardless if what's correct, my
aesthetic impression is that the first display, without m17n and libotf,
looks better; however, I do know that the Arabic example in HELLO is
correctly displayed only with these libraries).

Steve Berman