i'd love to try it, but my character copies are STILL in limbo and uncopied over to the PTR. i'm left with just a premade toon and unfortunately no pet biscuits. :(
if someone with biscuits could test it on the PTR, i'd love to know what happens upon log out/log in after using it on your vanity pet!
UPDATE: thank you rhapture for testing it on the PTR! unfortunately, pet biscuits used on the companion will not persist through log out. when you log back in, your pet will be waiting for you but they will be normal-sized again. :(
additionally, after a quick test on the PTR, pets seem to stay out when you zone as well (
strangely enough, if a vanity pet was present when hearthing, it won't show up after loading in. however, right after hearthing, i logged out and logged back in and the companion i had out prior to hearthing suddenly re-appeared.
in conclusion - it's buggy.