Apparently, this only works on the OS X and Linux versions of Skype. If you want to edit the last chat message you sent, you can use this syntax:

s/mistaek/mistake/

Skype will then update the last message you sent. However, the recipient will see a message indicating that it was edited. This is equivalent to right-clicking on the message and selecting Edit. However, for someone who uses vim all day, this Skype feature is awesome.