I am finding that somewhat debatable. Nids are so laughably high in number that they are visible from a stellar-level. The only things that the Imperium are aware of that are outside the galaxy is pissed off orkz, and nids. LOTS of nids.And Vong had worlds still producing ships,and sending them faster then Nids ever could.
Sufficient amounts of nids that a low-ball estimate suggested that the imperium would have to make it's war capabilities five times it's current level to stand a chance.
And given that the Imperium would curbstomp the Empire. Vong don't stand a chance in hell.