[kh99]

Hmm...well, here's a list of what's being posted when I search from the search page using "single content type" of post and selecting some forums:

titleonly=0
searchuser=
starteronly=0
tag=
forumchoice%5B%5D=5
forumchoice%5B%5D=6
forumchoice%5B%5D=7
forumchoice%5B%5D=8
childforums=1
replyless=0
replylimit=
searchdate=0
beforeafter=after
sortby=dateline
order=descending
showposts=0
saveprefs=1
dosearch=Search+Now
searchthreadid=
s=
securitytoken=1406582637-cf67eb3a60e2ff5376b1527b3643f4ef0f2351cb
searchfromtype=vBForum%3APost
do=process
contenttypeid=1

So I'm guessing that the forumchoice[] fields are only used if some other option is set to a certain value (but sorry, I don't have time right now to figure out which one - maybe you can experiment).

Edit: glancing at the list, most of them are just options that I wouldn't think would affect whether or not forumchoice[] is used, so it seems like maybe you need do=process, contenttypeid=1, and/or dosearch=Search+Now.