This post references Windows 10, but most of the operations are very similar under earlier versions.
-- Your Vendetta files are in "Program Files (x86) >> Vendetta Online". In this folder, create a folder called "plugins".
-- Most plugins are accessed via a link to a .zip file. For example, at
http://forum.tgft.org/forum/topic_show.pl?tid=5127, you will find a link to an an attachment "honk.zip".
-- Click on the link. You will be prompted "What do you want to do with honk.zip?" Click "Save".
-- You will then see a message "honk.zip finished downloading." Click "Open".
-- You will then be taken to the directory of your Downloads folder. In it, you will see a folder "honk".
-- Drag the honk folder into the plugins folder. The plugin is now ready to use.
-- tsreknor