It happened to me too lately, but I really don't think it is for all vectors (after that batch I submitted an other vector and they did not ask for anything).
Perhaps it is just a new reviewer for vectors.

I haven't had this issue, so I'm guessing it's a one reviewer clicking a random button to meet his/her rejection quota. My random rejection button this week is "keywords." Other weeks it's "image size" or "edges" or some other stuff and nonsense.

Oh, I can do it, even though it means more work for (what is now) less pay.

I just think it's pointless.

Anyone who can create a vector can produce a reference image, even if they didn't create the original image.   

If it was something that would actually protect against theft, it wouldn't bother me so much.  But, if someone copies a design to upload it to Shutterstock, asking for a reference image won't stop them. 

I feel the same way about this as I did when Wells Fargo asked me to show ID to make a deposit.

(iStock quit asking for reference images some time ago, once they figured that out.)

I already stopped uploading my hand drawn illustrations to Shutterstock when they insisted on a separate property release for each drawing.  A property release stating, "this is my artwork, and I am using it with my permission" was no longer enough, I had to attach a thumbnail for each illustration to a new copy of the release.   I was not really interested in creating hundreds and hundreds of those things.