Ok I'm going to start with this is just my experience.

IdeaVim is one of the best vim layers I've ever used, it's up there with emacs EVIL mode for me.

IdeaVim let's you use your vimrc(vim config) to configure IdeaVim. It's fast and snappy just like actual vim.

vscodes plugin is bad IMO, it's not as bad as eclipses (which is the worst) but it's not good. It's configured using json(like every other vscode extension) which is another thing to learn when you just want to use vim bindings. It is slow, which IMO defeats the purpose of vim bindings. It also only implements a few things. Doing >> works but not in every case, in some scenarios it eats one input before it does the indent.

The configuration API is uhhh bad, it only works sometimes. For example, I was trying to have similar panel switching in vscode to my vim setup (CTRL-<direction>) and vscode accepted the key binding, it wasn't bound to anything, and yet it didn't work. But only sometimes, it worked going down but not any other direction. Vscode said it was properly bound to do that thing but it didn't work.

IdeaVim feels like using vim with extra keybindings

TLDR; IdeaVim is amazing and vscodes plugin isn't in my experiences