Totally Not Corrupt Cop

I believe you can still play with both installed, but it is highly recommended you use different directories for each. The best possible solution is that you have three separate directories for each game (FiveM, LSPDFR/Mods, Vanilla). If you currently have a Vanilla copy of GTA 5, I would just make a duplicate and use that for LSPDFR, while you keep the Vanilla copy and use the other for FiveM. To explain more, you should have three copies:

Grand Theft Auto V
Grand Theft Auto V - Vanilla
Grand Theft Auto V - LSPDFR

Whichever one is named Grand Theft Auto V is the currently active one. You just need to change the name to Grand Theft Auto V to make it the folder you want to run. All the others should have a name as reflected above.
 
 
Hope This Helps! 🙂