Except for the Lawyer near the bottom who cites a wider array of laws including one that specifically says it's illegal
https://www.law.cornell.edu/uscode/text/18/1466A But we must not forget the interpretation of laws, their application, and the legality according to the constitution lies with the judicial branch. Previous attempts at applications of similar laws in situations regarding drawn explicit underage have been struck down.
As such they are usually reserved for exacerbating other criminal actions, violations of probation/parole, those actively involved in the military, purchasing/importing this content from other countries, and pressuring businesses to comply with standard views of morality (the pressure this site finds itself under from another). Attempting to prosecute for this law may see it struck; prosecutors lose leverage against real criminals and perceived immoral businesses additionally opening up the potential to sell real goods (manga) of the concerned content. The laws exists and serves only as an effective scare tactic.
I spent a day recently looking into judicial proceedings, and the transcripted opinions of dissenting judges of the supreme court, and read about of a few bloggers personal experiences. The first few lawyers are not well informed, the last has done his research and is accurate in his assessment. The risk is exists however slight, but minimize this by not breaking similar laws in conjunction. Do not forget that State and Local laws may apply.
On the plus side vore does not require a visual sexual component.
(I took way too long to type this up notice the reply above me agrees)