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.