As this is a new question I've split it to a new topic. For reference, when starting a new topic please click the "New Topic" button in the section of the forum that is appropriate to your Audacity version and operating system. For example, if you are using Audacity 1.3.13 on Windows, go here
and click on the "New Topic" button.
To apply an effect to only the vocal, you need the vocal to be on a different Audacity track to the backing music.
Depending on the format of your backing track you may be able to import it into Audacity. If the backing track is on a karaoke CD, you could record the backing track from your karaoke player into Audacity.
When the backing track has been imported or recorded, record your vocal. Your vocal will then be on a new track and you can add effects to it without affecting the backing track.