Hey Dan!
How I approach backing up is like this:
I create a music production folder which holds everything related to my music production.
Whether that be one-shot individual sounds, VSTs, the actual project files, etc.
To answer your question about backing up VSTs:
For FL Studio's VSTs, there's no need to back those up, as they come with FL Studio. However, if you've created custom presets for these VSTs, you'll want to make sure you back those up!
For third-party VSTs, I keep these stored in that main music production folder, which is stored in Dropbox for easy access/backup purposes.
My course "A Specific Music Production Folder" breaks down my music production folder more in depth. It's a quicker course!
Let me know if you want further clarification!
