It seems like you have a lot of questions regarding specific sounds or styles, and asking us to pick them apart may take a long time between replies. Luckily, there are actually better resources out there for these kinds of questions!
Let's address VSTs first.
Sound hunting is hard, and even companies that create Karaoke versions of tracks have difficulty matching a sound, and sadly, not all of us are wizards of waveforms. The best thing you can do is see what free VSTs have to offer you, and not discrediting them because they are free. Popular dubstep artist Savant actually creates most of his sounds in FL Studio with free, already included software such as 3xOsc. It might seem a little daunting at first to mess with these knobs, but google "3xOsc tutorial" and you'll find tons of guides on how to get great sounds using just the basic package! Starting out can be rough, but playing with those settings for a bit will make you think, "pff, who even needs the sound [specific song] had? This sounds way better!"
While I can't address your Megalovania questions, I can say that 3xOsc is a great place to start - if you want to make 8bit sounds, you might also want to look into magical 8bit plug.
If you feel like you've reached the limits of what you can do with a particular plugin, try using filters! Filters can manipulate a sound to pretty much be anything. Think of filters as "effects". Want to make your piano sound like a guitar? FL Studio's Hardcore can definitely do that for you. Here's a video I made where I just mess with filters to make a single free piano plugin (PianoOne) sound a little more interesting (I even skipped to the actual track for you )
Someone earlier in the thread linked to other resources that I think are worth checking out, but I definitely recommend seeing what Sonic Academy has to offer: https://www.youtube.com/channel/UCS972vXt4jnzkMlvHngalEg
Also, I apologize if it seems like I'm demeaning your music creating experience by telling you to go back to the basics - but sometimes, it helps. Pre-made samples are nice, and using another person's instrument tone is a great place to start, but all of this starts with a little bit of self-research. Playing around in oscillators can help you understand how some sounds are made, and looking at tutorials will help you master how to get the sounds you want out of your DAW/VSTs of choice! Heck, even Toby Fox himself (the person who wrote the track you last linked) has a FL Studio tutorial that covers some basic things here: http://tobyfox.net/Tutorials/FLBasicTutorial.html
TL;DR: There are likely better resources than OCRemixers - while we're happy to help, Youtube will probably be your best bet in learning how to get the sounds you want. (start looking through Sonic Academy uploads! https://www.youtube.com/channel/UCS972vXt4jnzkMlvHngalEg)
Anyway, post things you make in the Post Your Original Music! section of the website, receive critique, improve, repeat.