Do I have to provide better support when hanging a range hood from soffit framing?
Every big box has said hood you need.  Do they have one that is fancy or that you will like in the depth.  Who knows?  

As you can hang your "vent" from anywhere that is secure.  Note that whether you have to vent out is up to local code.  Most don't require it, meaning if they don't you want to think about not having anything - what you are getting is less effective than a ceiling.  You might also want to think about just installing a ceiling vent/fan (bathroom vent variety).