VSCode Vim extension binds <Ctrl-d> for its own use. This keymap is used by VSCode for “selecting next word occurrence” in a multi-cursor fashion. If you like VSCode to handle this keymap (and others of your choice) you could do so via vim.handleKeys option.

The following table is copied and pasted from the project:

Setting Description Type Default Value
vim.handleKeys Delegate configured keys to be handled by VSCode instead of by the VSCodeVim extension. Any key in keybindings section of the package.json that has a vim.use<C-...> in the when argument can be delegated back to VS Code by setting "<C-...>": false. Example: to use ctrl+f for find (native VS Code behaviour): "vim.handleKeys": { "<C-f>": false }. String "<C-d>": true

Using the vim.handleKeys option you could delegate the handles to VSCode by setting each to false like so:

"vim.handleKeys": {
    "<C-d>": false