How to add a transition to all layers in VSDC video editor instead of having to it one by one?
VSDC team member is here. If you're talking about the transition effects, you can add them using the wizard. Although, you'll still have to drag each one for each transition. 

However, if you need to copy any other effects, we have a how-to video for you: How to apply the same effect to different objects with VSDC Free Video Editor. 
I hope this helps!