I found out that you can debug Vanilla Minecraft with IntelliJ recently. Heres how
1) Go to the Minecraft Launcher and add -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5009 to your JVM arguments
2) Launch Minecraft normally
3) In IntelliJ go to Run > Attach to Process
4) Select net.minecraft.launchwrapper.Launch
Hope this helps someone