To play online you need to have steam, Payday 2 demo and Steamworks fix apparently.
Skip step 2A if you are using Payday 2 and follow step 2B
Follow step 2A if you are using Payday 2 Demo and skip step 2B

1: Copy and paste the content of CODE.VEIN.Online-Fix into the installed game folder.
2A: Launch Steam and download Payday 2 demo. You need Payday 2 demo in your game library.
Download it and it will be added into your library. You don't need to wait for the download to be done. Pause it after you click download.
2B: Launch steam and you need to do some minor tweaking.
Go to Code Vein\CodeVein\Binaries\Win64\cream_api.ini
Change newappid = 251040 (Payday 2 Demo) to newappid = 218620 (Payday 2 game ID)
3: Launch CodeVein.exe
Online play only with Crack Users and people who have the same Steam Download Region.
Launch Steam - Top left click Steam - Settings - Downloads - Download Region
RESTARTING THE PC MAY BE REQUIRED FOR THE MULTIPLAYER TO WORK
Save file location changes depending on the steps you took to modify the game.
By default (a fresh install without applying the steam fix) it goes to:
C:\Users\#####\AppData\Local\CodeVein\Saved\SaveGames\1638
If you already applied the steam fix on the game it changes to:
C:\Users\#####\AppData\Local\CodeVein\Saved\SaveGames
If you launched Steam + Payday 2 demo + steamworks fix:
C:\Users\#####\AppData\Local\CodeVein\Saved\SaveGames\130826016
It's better to save a backup of your save file in case you've overwritten the wrong one.

For the DLC to work you need to tweak a file.

Go inside your installed Code Vein folder and go to Engine\Binaries\ThirdParty\Steamworks\Steamv142\Win64\steam_emu.ini
Change DLCUnlockall=0 to DLCUnlockall=1

If there is no audio when playing the game or when joining a distress signal:

Go to Control Panel\Hardware and Sound\Sound disable other playback device except the only default one you are using

Credit/Source : Aceofgods (online fix guide), uzkarter (steam fix link) and ReborN (Payday 2 gameID) for posting help on the online fix.