| What is your definition of “vented to the soffits”? If the ducting simply ends inside at the soffit/eve, which it sounds like from your post, you could eventually have problems. Soffits have vents for air circulation to the attic, they are not made for venting bath fans, or any other type of exhaust fans. I’ve installed a number of these fans and all the instructions, and everything else I’ve read, state the fan should directly vent to the outside, for all the reasons TKRussell stated. You don’t want moisture in the attic.
Or, is your fan actually vented to the outside at the soffit with an under the eve vent, as opposed to other houses in your area that vent at the wall? These under eve vents are used when it is not practical to use a wall vent, or if you do not want to cut a hole the roof for a vent. The problem with these under eve vents is that the ducting has to bend downward at the vent connection which reduces the fan efficiency.
As TK stated, may be best to give your local code department a call… | 
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