John Castell in Slate writes an article as a follow up to the Virginia Tech tragedy about "The Gun Law That Would Make a Real Difference" even as he admits that, in this case, it would make no difference in this instance. He does not mention the change that might have prevented the VT tragedy.
In 1998, the Brady Bill (which mandates the background check) was revised. Previously, any purchaser at a gun shop had to wait five days to purchase a gun as a thorough background check was performed. However, the 5 day waiting period expired in 1998 and was replaced by the current instant check system called NICS. This instant check failed to pick up Cho Seung-Hui's involuntary stay at a mental hospital, which should have disqualified him from buying a gun.
The "Gun Show Loophole" is also an inconsistency in current law, but it is not the problem that allowed Cho Seung-Hui to purchase two guns.