hi vikispod! welcome!
something important to keep in mind since you mentioned VSTs is there are only a few formats where you can just submit an .mp3. they are:
allgear,
wildchip,
fakebit,
remix and
vocal. "allgear" (which is basically anything goes) and "wildchip" (same, but with chiptune-sounding things in it) are run fairly often since they're accessible for everyone. but for everything else, hardware-compatible files like .nsf for NES formats, .mid for MIDI formats, .vgm for lots of old FM synth and PSG soundchips, etc are required.
there are some good VST options both free and paid if you just wanna do the allgear/wildchip formats - i'd say check out
Magical 8bit Plug for the NES sound,
C700 VST for SNES (which can do proper chiptune exports too
with some setup),
Genny VST for Genesis (
likewise), or you could just use a sampler and download samples! or a soundfont player like
Sforzando or
TX16Wx since people have dutifully compiled soundfonts of a btrillion old games' sounds at places like
this or
this. those are free options - if you are willing to pay, Plogue's entire
chipsynth line is extremely good, and
Super Audio Cart is a great sample repository.
FMDrive is a paid Genesis option too.
a few formats here do work with VSTs but require something other than .mp3 submission - i mentioned already that C700 can do proper SNES exports and Genny can do proper Genesis exports. similarly, MuntVsti for
MT-32 MIDI and S-YXG50 for
XG MIDI are VSTs used in a DAW to sequence and hear MIDI playback for those formats, but you have to submit a .mid file. we have instructions for how to do all these things on our Lyceum which is like our wiki.
... for everything else, there's learning to use trackers! tracker software is the main thing required of many formats to get the proper filetype exports, and learning those will start really unlocking more formats for you here. you can have plenty of fun without them but diving into trackers will open things up fully : )