Depends on what game you're playing. Check the rules for the game, if they don't say anything then make a forum post (or if they have a Discord server you can ask there).
Exactly that ^
However I will say that unless the game just doesn’t care about accuracy, VBA is almost always banned. It’s a very inaccurate emulator so most communities don’t allow it.
From my personal experience switching between emulators and Gameboy Player a lot, Bizhawk (with either VBA-Next or mGBA core, never noticed a difference for RTA here, but I had a TAS desync on VBA-next, while mGBA does not seem to accept randomized roms (even after adding the checksum to the gamedb textfiles)) seems to be the more accurate one, although for some weird reason the Pokemon community still requires VBA (or VBA-rr iirc) even tho anyone who ever did R/S/E on that emulator knows that the input lag is way too much for precise mach bike movement, which is already hard enough as is.
TL;DR: If there's nothing forced in the game's rules, Bizhawk is the better choice imo, because VBA has stupid input lag.
Hmm, I went by any guidance spreedrunslive offered: "Banned: GBA - VBA-M (use VBA instead.)" Perhaps I will try Bizhawk going forward.