Last parameter of last command in bash in vi-mode
There's no default. The binding for 'yank-last-arg' (as listed by 'bind -p') disappears when you switch to vi mode.
bind '"\e."':yank-last-arg
will give you that same binding back (or pick something else)
There's (vi-yank-arg), by default mapped to "_". That should do what you want (in command mode).
Not exactly the same, but in either mode you can type !$, and it will be replaced by the last word of the previous command. Find more such things in the manual.