The seller and auction seemed legit, so if it is bootleg, I think the seller might have thought it was legit themselves.
I became suspicious when I saw the tag was missing the blue sticker. It could have been removed though. The tag itself looks legit. It's made of the same material as other banpresto tags, and has writing on the inside too like other tags, but when comparing it to the tags shown on this collection site: http://hantsuki.weebly.com/umbreon.html (if the owner doesn't want it posted let me know and I'll remove it) there are differences. Most disturbingly, mine says "best wishes" instead of "diamond and pearl" on the tag's back. It also has a 4 instead of a 5 and some of the Japanese text in the front is shorter. Since I can't find a lot of info on the plush, I'm thinking there might have been more than one version, but being a Holoween plush, I'm doubtful. I'm also a bit suspicious of the embroidery quality. Here are the pictures of mine click to see full size:
Here's a quick group pic of my collection. You can see I started a small new collection! After buying some of the Chandelure merch from the Mystery Mansion stuff I just had to start a new collection. Thankfully He doesn't have as much merch as Umbreon!
I'm planning on taking some detailed pics for a proper collection update, but I just haven't had a lot of time lately. I hope to soon. I got the bootleg shiny for Christmas LOL, but he fits well :)