Ok i just tested a theory, after you install items from the vault into you’re project you can then remove local content under the vault page in your Epic library if they are not needed be installed in other projects.
This way you can save drive space on C where Vaultcache is located and can’t be moved.
You just install into whatever project the files will stay in that project on whatever drive you have it installed to while you can then free up space again deleting all those files just by hitting the remove from local content tab in the Epic Library where all you’re vault content is stored.
I just found this out and it’s now saved me massive amounts of space on my main system drive.