DON'T CLOSE VBA-M UNTIL YOU SAVE IN THE GAMECUBE GAME (NOT SAVESTATE, SAVE). Unlike in real life, once Dolphin and VBA-M are linked, they are linked permanently.SAVE BEFORE YOU DO IT. I don't know how many times I did something in Wind Waker only to lose all the progress because Tingle decided he was going to fail to connect and take Dolphin down with him.However you are definitely going to need to not be at max graphics in Dolphin since you are running two emulators at once. This is normal so don't worry if the framerate tanks in Dolphin during connection. It's going to kill the framerate during initial connection.Just keep pressing connect, or whatever option you use in the Gamecube game until it connects. For example in The Legend of Zelda: The Wind Waker, sometimes the Tingle Tuner won't say your linked even though VBA-M is open. There are a few things to note when doing GBA Link Cable Emulation (Will test this to see what still holds true for version 2.1.4 of VBA-M, give me a day or two I'm busy): Then do whatever you need to do in the Gamecube game to cause the link to occur and you will be linked.Repeat steps 6 and 7 for however many GBAs linked to the Gamecube (max is three, don't go over).In VBA-M, go to File > Open and open the Bios file.Open VBA-M and go to Options > Link > Type and set it to Gamecube.Run the Gamecube game you want to use the GBA Link Cable on in Dolphin.Go to Boot ROM and set the directory to the Bios file. In Options > Game Boy Advance > Configure. In Options > Game Boy Advance Use BIOS file must be on.
In Options > Link and turn on Local mode and Link at boot. In Options > Link > Type and set it to Nothing. In the Emulation settings, Pause when inactive must be off. In the Emulation settings, Skip Bios must be off. Again setup the controller and other settings however there is a few things everyone needs to make sure are set right in the settings. Set multiple ports for multiple GBAs to be linked.
Here is a guide on how to dump Gamecube and Wii discs.
(Link to main Github page for instructions on compiling on Linux or if you so choose to compile from source code yourself)
Dolphin is only a 64 bit application so make sure to only download 64 bit VBA-M)
(General releases for when I don't have time to update the direct downloads. Here is all the versions, get which one matches your operating system. You can get builds here (I mean, what else would you use as a Gamecube emulator anyways.)