V'Ger's cloud is 2 AU in diameter (going by the Director's Cut), which is the same diameter as the Earth's orbit, 300,000 km. Fitting inside the cloud is not a problem.
Actually, 300,000km is the distance light travels in a second (roughly), not two au. An au is 149,597,870km. Two astronomical units would therefore be about 300,000,000 - three orders of magnitude more.
D'oh! Of course I knew that. My brain skipped a few steps for some reason.
It happens to the best of us.
