Vaal power siphon doesn't seem to be able to split via Sniper's Mark or Fury Valve...

...despite it working as expected with chain, fork, etc (which is the main thing that makes me think it can't possibly be intended) and it also has the split line in it's tooltip description

Screenshot of the skill tooltip and bug report number:

Screenshot trying to show that it doesn't split at all, kind of hard to see in a screenshot but yeah, the image is taken after the skill has hit, and there are still enemies left over that should have been split towards and killed:

Normal power siphon of course works as expected and splits, it actually results in the vaal version having far worse clear compared to the normal version when using split other than when there's a very large amount of enemies