Have you tried changing the woocommerce settings? I’m not sure that Wc Vendors has anything to do with the product image sizes….
On your admin dashboard, try going to:
Woocommerce > settings > products > display
There’s a section titled “Product Images”
Is that what you’re looking to change?
Otherwise, some themes have the images sized in the css files. you’d have to look for the woocommerce folder for the the css files (sometimes under assets in your theme files) and copy the image files to your child theme for modifications.
I hope this helps…