Heather,
I believe you are going to need to address this on the server side.
I do not see anything on your site that should be so greatly affected by the plugins and the storefront theme.
Somehow the normal script is being blocked.
We have over 20k sites running WC Vendors- occasionally issues do arise, and we are happy to address them and fix issues if they are in our program. That’s why we are here.
I cannot discern why your install s are not handling the uploader for images, and why the script is being blocked from time to time to prevent even accessing the uploader. 🙁
I think I have gone as far as I can- all I could assist with at the time is advising you through swiping your install and starting fresh with ONLY storefront, woocommerce, wc vendors (configure it) and then wc vendors pro.
Only then- after those is configured and properly working after testing, would you add any other plugins or design elements to the site.
If storefront, woocommerce, wc vendors and wc vendors alone are not working on your site, then there is something wrong on the server end. The configuration of these three plugins with storefront is solid.