I've tested the Moana cloud some times ago in Corona (for Max), and while it was slow - I agree - it looked quite good.
There are a couple things to keep in mind for a fair comparison. Do you have similar phase function settings across all renderers? This is one of the most deciding factors for how realistic rendered clouds will look. The 0.5 value in Vray's examples will be typically too low to generate dark edges, a more realistic value is in the range of 0.7 to 0.9.
Corona's performance tanks once you use higher values for the phase function (positive values mean forward scattering) and it'll be quite slow for values close to 0.9 - generally increasing this value will result in slower rendering since way more samples are needed to resolve the scattering within the cloud.
Another factor is how many bounces you use. Corona's default will be way more than most renderers. However for a realistic result you'll need to go even higher.
Also... what's the 'other renderer'?