I don't know that there is a collection of them (probably somewhere), but a few of the common ones are:
OT = Off Topic
FWIW = For What Its Worth
ROTFL = Roll On The Floor Laughing
ROTFLMAO = Roll On The Floor Laughing My A** Off
Then there is always a bunch of terms/acronyms that have one meaning to one group and a whole different meaning in another group.
I don't think a rigid definition would mean much. If a post is designated as being Off Topic, it means the sender acknowledges that the subject raised is not directly related to producing/editing video using Vegas, which is why this Forum is here.
The line between On or Off, however, is somewhat wavy. Many of us have been here for quite a while and we sometimes share topics that we hope will interest others, even if they're not directly about video production.
AFAICT = as far as i can tell
WOT = way off topic
OT = on topic (not to be confused, somehow, with OT = off topic)
LOL = laughing out loud
HTH = hope this helps
HAND = have a nice day
BRB = be right back
AFK = away from keyboard, and by extension, not online for a while
IANAL = i am not a lawyer
and then there's ...
:-) = smile
:-( = frown
:-\ = uncertain
;-) = wink
>:( = very mad
:-D = laughing
:O = yawn / shocked
:o) = big nose
x-) = sleepy
8-) = surprise / too much coffee
:B) = looking over your glasses
3:o) = moose
:() = duck
o>{O: = penguin
?:-) = Ronald Reagan
******:\ = Marge Simpson