I was using my points to collect one of each of the plot items. I bought them in two "batches"... the first batch was basically all the books, and a few other items (such as the pyjama set). These were then sent from my inventory to my vault via the Quick Stock feature in my Reika's Hoard of Goodies shop. I then went back to start buying the rest of the items. I worked my way up from the bottom of the shop listing to the top and stopped just before buying the first row. I then used the same Quick Stock feature again to send these all from my inventory to my vault (I also had a few other items in the inventory, such as a token and some items obtained from the Free Shop).

I then wanted to be sure I hadn't purchased doubles from the plot list, so went to check my vault. Before actually checking the list I went to create a list for "2018 Plot" in the vault, so as to be able to see what I'd already had without seeing anything else in the vault. I then went to the items section to begin adding the items (including the extras like the coffee run and the snowflaik in a bag) to this new list. After adding the ones I found in the vault, I felt that the listing was a bit short, so went to check again. I then noticed that pretty much every item I had just bought from the plot shop was missing, as were the few other items (not plot related) I had added from the inventory to the main vault list.

I did recheck the inventory in case they simply were still there. I also checked the shop I'd sent the items through (via Quick Stock feature), and even checked my other shops just in case I'd clicked from the wrong shop. I even went to check the donations area just in case I might have inadvertently sent them there. While there were quite a number of donations, none were from me (and believe me, I refreshed a lot to check). All but a few of the plot items I bought are definitely missing from my vault.

The only items that appear to have made it safely into my vault were: Dungarde Chain, Godfrey Skelly Plushie, Grave Dirt Grub, Leftover Morostide Candy, Pink Crystal Beanbag, Pocket Magnifier, Rotting Lace, and Ruby Widow Ring.

Unfortunately, other than a token (which was given from the random oracle's quest), I can't remember what all the items I got from the free shop that are missing as well.

EDITED TO ADD: I just did a test to see if the quick stock was glitching... I tried to send a black trenchcoat to my vault via Quick Stock from my Reika's Hoard of Goodies shop. It did not show up in the vault, nor is it in my shop or inventory.

Hello! Do you know if this glitch is still happening?

I'm not sure... I've been too afraid to try again. I'll go find something inexpensive and try it for you now.

EDIT TO ADD: Just popped an item (Kanis-Foot Slippers) into my hoard shop, then followed the same method I did before to move it to my vault. Seems like this time it was sent fine. Guess I was just the unlucky recipient of a glitch. Thanks for checking for me though :)