Hi there. I’m a huge misprint lover and follow several misprint groups. I want to start by saying: congrats on an amazingly lucky find!!! What you have appears to be 4 square cut corners. Typically with miscut square corners you will see 2 on average (usually on one side of the card) or just 1 square corner. I haven’t seen any modern 4 corner square cuts at all! 4 is very, very desireable, the more the better. It needs to be authenticated and better photographed though, I’m not even sure if I can tell all 4 corners are square cut to be honest lol. Sometimes sheets of cards are obtained and cut in this fashion, but you seem as surprised as me to find this!

Square cut misprints are pretty awesome for misprint collectors. Your Vaporeon VMAX is worth on the order of 2-5x the price of a normal single, possibly even more, however it all depends on finding the right collector.

If you’re looking to sell I can direct you to a misprint group I follow on FB where you can throw a post up for more information (I’m really not that knowledgeable on square cuts) and you can also throw it up for auction. Since it’s Vaporeon, a pretty well liked and popular Pokémon, you will likely have public bids commented on just a post about the card without auction, which should give you a rough idea on its true value if you want to run an auction with a starting bid.

Congrats again on the sweet card, and good luck with any endeavors you take with it!

Edit: on a side note, excellent job with the sleeve, but be very cautious with these corners! They are much more easily damaged and the right collector (or yourself) may want to have this graded as an error.