MAX 7+sp1.
If i render in Vray VFB whit separate channel - Z, coverage, velosity, render id - this channels NOT saved.
I can see this channel in VFB. I can’t save this to file. File is a black.
If i render to RPF - all ok.
I very want use VFB. Whats a problem???
These channels are not saved, because there is no standard RGB format that allows them. The only way to save those channels would be to save to a .rpf or .rla file from the standard 3dsmax VFB.
Best regards,
Vlado
Vlado you not right 
Combustion 4 have operator - G bufer builder.
This operator have channels - velocity, z buffer, coverage …This channels can load as RGB files…
i assume if you use a color wheel to determine distance a pixel will travel and direction it will travel in. so if a pixel is a certain color it means its going in whatever direction that pixel would be in if it was on a color wheel. and by its saturation it would show how far it would move (this is just me best guess)
@HCafe : beware…Vlado knows what he´s talking about 
i assume what he meant is that the vray gbuffer channels are not limited to rgb image-depth. just take a look at the z-buffer…it reflects true scene-depth if i recall right…wich is floating point accurate…hence wont save to a standard image…would have to be either clamped or scaled (and henced reduced accuracy)…
Correct me if i´m wrong here hehehe
Besides as Vlado noted already if you save it from a standard vfb to rla/rpf you can of course use them in combustion…but give it a try…it´s in-accurate as hell and in most cases it´s not antialiased so it´s a b*%§/ to use
As of how to store velocity, it´s just rgb = xyz defining the direction and speed…
Thorsten