How to run Venice Unleashed on Linux (and Steam Deck).
UPD. Launch Guide Upadated. Check new method Below!

Required:
- licensed copy of the game Battlefield 3 (steam/origin). I used the license on Steam
- lutris
- browser with user-agent switcher extension. I used Google browser

Steps:
Step 1) install the game
1.1) if you live in the Russian region, as of 03/27/24 there are difficulties with launching the RU version of the game. In the game properties, change the language to English.

Step 2) install the browser. In the extension store, install the extension "user-agent switcher"

Step 3) launch the game, wait until all components are installed. Log in to your EA account. When launched, a browser will open with the Battlelog page, log in to your account. In the browser, click on the extension, set the Firefox -> Windows Firefox option. If you want to install this option NOT ON ALL pages, but only on battlelog, go to the extension parameters, then to the pernanent spoof list tab, add a link to battlelog in the Domain field, select Windows Firefox in the User-agent String window, and save.

Step 4) download the punkbuster update installer for windows from the browser and install (I installed it by adding it as a third-party in steam. You can install it differently, there are guides on the Internet and on protondb)

Step 5) install lutris, configure it as you want (guides on the Internet). In lutris itself - click on the plus -> Add localy installed game -> in the "name" field write what you want, in the "select a runner from the list" option put Wine.
In the "Executable" field, I first put the path to bf3.exe (located in the directory with the game installed). In the "Working directory" field - the path to the directory with the game installed.
In the Wine Prefix item, select the path to the directory with the game prefix. In my case, the path is something like this - "/path/to/folder/Steam/steamapps/compatdata/1238820/pfx/".
In the "Runner Options" section I chose proton experimental (the same one is available in the game itself on steam, chosen by the Valve team). I personally did not touch the remaining options. Save

Step 6) then click on the up arrow and click “run exe inside wine prefix” and run the downloaded “vu.exe” and install as usual.

Step 7) Next you need to replace the executable file. To do this, go to the directory with the prefix, but go deeper - /.../pfx/drive_c/users/username/appdata/local/veniceunleashed/client/vu.exe", copy the file location and add it to the "Executable" field

Step 8) F̶r̶e̶e̶d̶o̶m̶ then you can make a shortcut in Steam and launch it without going into lutris. Launch, log into your vu account, play

To my surprise, the method of launching Venice Unleashed on Steam Deck is probably outdated, and now in order to launch this crap and make it work, you need a degree in dancing with a tambourine. Having not found any materials (I searched as best I could, I'm not an expert) and good alternatives, I solved the problem radically. I installed another steam client via portproton, installed bf3 there, and then everything according to the instructions. Of course, this adds weight to the installed Steam, which is needed once to launch a licensed copy, but either this way, or I don't know. Purely technically, the second steam should not harm either the system or the native steam (well, if I understand everything correctly), so there should be no problems. The method via Lutris and adjusting it to the Steam Proton now does not work for me either (either something went wrong with the Steam updates, or with the Lutris update).
In short, what I did:
1) Downloaded the Steam installer for PC from the site.
2) Installed Steam, logged into my account, downloaded the game (in the "store" section there will be/maybe epilepsy, but in the "library" section there will be no flickering, and nothing else is needed).
3) Launched the game, installed VU in the prefix, then RMB on the PP icon in the tray and pressed "File manager". Then a window opened in which I went to drive C:, and along the path "/home/deck/PortProton/prefixes/DEFAULT/drive_c/users/deck/AppData/Roaming/Microsoft/Windows/Start Menu/Programs/Venice Unleashed/" opened the file Activate BF3.lnk. The console will open and say that VU has been activated, and everything is fine, after 10 seconds it will close itself. Then you can turn off Steam, turn off the battlelog, we don't need them anymore.
4) I followed the path "/users/deck/AppData/Local/VeniceUnleashed/client/" (on the deck itself, it's in the portproton prefix), pasted the bat file there (since PP can't launch several applications at once, I made a bat file that launches the local server and the game at once), called it "launch.bat", and inside -
@echo off
cd "/home/deck/.local/share/Steam/steamapps/common/Battlefield 3/client/"
start vu.com
cd "/home/deck/.local/share/Steam/steamapps/common/Battlefield 3/client/"
start vu.exe
(I just now noticed that the old directory address remained here, which differs from where the files are located according to the new method. And the bat file still works, so I'm screwing up somewhere here piled up, but if it works - it works, don't worry)
5) Right-click on the bat file, "open with", look for PortProton, open, select your prefix and the previously used proton (I always have the same default ones for everything), for convenience I created a shortcut, added it to the desktop and to Steam.
6) Launched, the server and the game started, and everything works
Workability in game mode has been confirmed! Everything works in desktop mode.