how to combine animation with animated background in VSE?

Hi.

I’d like to combine (rendered) animation (or several animations, or cycled animation) with some another (avi) animation in background.
Say, like an animated character walking on top of live street movie. Or an animation with live filmed clouds instead of rendered sky.

Using compositing nodes should work, but will require to render repeated animations several times.
Is it possible to do in VSE with some special kind of render of foreground animation?

In reallife filming they use chromakey.
But idea of remmoving background from render looks not good.
Rendering with alpha-channel is only possible into still pictures, not in avi.

If i recall correctly, blender can render separate diffuse/zdepth/other stuff separately, instead of post-processed video.
Is it usable in VSE, or only in composing nodes?

It is perfectly normal to render to a sequence of stills with alpha background. Then you can composite your sequence in the VSE using an ‘alpha over’ blending mode.

VSE is (mostly) used for assembling the rendered/raw/whatever material into an image sequence or movie. That’s the general idea. It can be used to do other stuff with varying degrees of success. In principle you can use e.g. image sequences of live clouds to composite over cg or vise versa using simple blending modes so the short answer to your question is yes. A more elaborate answer, however, would highly recommend the compositor because it was designed to do the things you describe in your post with absolute control over every aspect of the process.

is it possible to use compositing nodes to recombine already rendered videos?

yep, that’s the general idea though one would most probably use still instead of videos. But then again, it all depends on what one needs to do.