Does anyone have any idea how I could keep a modded version of GTA and LSPDFR version at the same time and be able to play any without re-downloading everything?

What I have done is to make a new folder where GTA5 folder is and copy all contents of the vanilla gta into my new folder and named that folder "gta5 single player". I then get all my mods and drag them into the "gta 5 single player" folder, copy ragepluggin and make a shortcut too desktop.

 

Yes. Use OpenIV, go to Tools > ASI Manager. Install both plugins. 

 

Then go to your GTA V directory, create a folder called mods and install all the game file mods there (script mods still go to the normal directory!).

You can find a detailed instruction here: http://openiv.com/?p=1132

 

Whenever you need vanilla GTA, just remove the plugins from OpenIVs ASI Manager and the game will load the vanilla files :)

I have 2 gta 5 game installs and if i want to play multiplayer i only need to rename one