Use Extendscript to add a (adjustment) layer to a comp?

I'm trying to programmatically add an adjustment layer to my comp. The adjustment layer creates a vignette effect, and is inside another comp inside another project (vignetteProject). Here is my code:

var vignetteComp = vignetteProject.item(vignetteIndex)
comp.layers.add(vignetteComp.layer(1))

On the second line here, I get an error reading "unable to call 'add' because of parameter 1. [object AVLayer] is not of the correct type". 

I believe it is expecting an AVItem, not an AVLayer. But if that's the case, how am I supposed to add an adjustment layer?

Thank you!