As the title said. Minecraft maps are probably one of the main reasons as to why I don't play on vanilla; everything is too blocky and there is little color contrast (Its either bright, or extremely dark, nothing between).
Because the maps don't require the Counter-Strike Source textures to be played on normally, unlike Dolls, for example, where some objects become invisible/purple and black.
Vanila maps are only minecraft simply due to the fact that they are vanilla maps. Not everyone has the textures needed or can run modded without experiencing some sort of lag, Vanilla is a nice alternative for those who don't want to play on our modded servers with those weapons and maps.
Which is sort of where I'm confused; are there no other textures but minecraft textures that are 'vanilla'?
Not to my knowledge but I may be incorrect on that matter. To play on modded you need CSS textures and Minecraft doesn't require you to have that, It's pretty much as simple as it sounds.
There are. Gmod comes built in with half life 2 textures, though many map makers do not build using only half life 2 textures because those textures can be very limiting. Counter Strike Source allows a map maker to use both Half Life 2 textures and Counter Strike Source textures. This is why on many maps you will notice that you are able to see a few of the textures but not all. Examples of maps on modded that only use half life 2 textures: Camel (correct me if I am wrong) Forest Canyon Island 2013 (I think it uses a CS skybox though) In addition, Minecraft has textures that are downloaded straight to the client when they download the map so anyone who has GMOD can play vanilla and see everything.