myusername,
I wear my impact vest above the harness, so impact vest will not help your case.
Impact vest protect against impacts

, although i manage to break some ribs while wearing it...

. I cannot remember if the pain appeared during the take off (similar to your problem) or at the landing (err... shall I say "heavy crashing"

). I think that a very tight harness associated to a powerfull take off was the cause of the broken ribs. A bad and harsh landing on the ribs might have finished the job

. I now do not excessively tight the harness to allow it to move a bit before it has to crush the ribs (if that makes sense

).
Hope this makes sense.
...or maybe your harness simply does not fit your body shape

Enjoy!
Fred