I've never used a mdp to vga adapter, but I can confirm the gl502vm/k supports 4 displays simultaneously with other adapters.
there are 2 displayports... one mdp, the other dp over usbc.
Getting a working adapter requires luck, as some active converters (like your vga) are just not compatible.
Are you using a monitor with only a vga port?
If you can avoid it, get a mdp-hdmi adapter instead. They are cheap, and can be had regularly for a few cents on ebay. This is the one I have, and I believe it is a passive adapter and does full hd just fine.
I also have a usb-c hdmi 2.0 active adapter. The adapter chipset is Via VL100 paired with Parade ps176.
these are the ones I can confirm that work. There are a bunch of usb-c hdmi adapters that are only hdmi 1.4.
You can also use a passive usb-c displayport adapter.
As for thunderbolt, you need to update your software and firmware from the asus website for your laptop model.
If your bios has a thunderbolt entry, you have thunderbolt.
I bought a passive adapter from ebay which doesn't work. I'm not sure why and the seller said he'll send another one since it seems to be a defective part. USB 3.1 is there at the USB-C port so I think that a USB-C 3.1 adapter to video output (vga, hdmi, dvi or mdp) can be used, although I haven't tested it. As for Thunderbolt, I can see just an option at the BIOS (User authentication, Video Output and another option), but when I install the software and check the details, nothing shows up. Also, there's nothing regarding Thunderbolt in the Device Manager. So I think my Unit has no Thunderbolt and only the BIOS has some kind of option since it suits both models, I don't know. The reason why I nees VGA is sometimes I use a second monitor at school which only has VGA port.