Easy to "vet" somebody when you know nothing about them and when you enter some sketchy info into some computer database thats also, miraculously, empty, again, its quick to get done... "oh, nothing here, must be good to go"...
On the other hand when the .gov has to search through tons and tons of info its been gathering on its citizens for decades...well now, that takes some real time and effort. Gotta search through texts, phone recordings, social media history, credit card purchase history, etc...
And I bet they have more people "vetting" refugee's than the entire ATF NFA branch has employees...by a factor of 10...at least.