There is a really simple, though tedious solution that many production houses use: Render your particles in scan line, and your background animation in Vray. There are TONS of effects that Vray isnt very good at rendering (because they were all designed to work with Max's pathetic scan-line renderer. I render most of my animations in Vray, but when there is an effect I need (lens flare, animated clouds, PARTICLES, etc) I use the scan-line renderer, render as a .tga or .rla sequence, and merge it back with my animation in After effects. Its more work, but usually worth it in the end.
-Nick