The Vim plugin in JetBrains Fleet uses the Vim engine of the IdeaVim plugin. You can view the full list of currently supported functionality in FL-10664 in youtrack.jetbrains.com.
Enable Vim emulation
Press Ctrl0, to open settings and select General.
Select the Enable Vim emulation checkbox.
Editing modes
With Vim, the caret is a block when you are in the Normal mode:
To change to the Insert mode, press i, and the cursor will become a line: