How do I modify a vanilla block in a mod

You can't modify vanilla code and it isn't allowed. However, you can create a GUI and then, you make a procedure to check when the player right clicks a bookshelf and then you open the GUI you want. It won't be allowed to store items when you close the GUI because bookselves aren't tileentities

You can modify vanilla code, and it is allowed. Lol. You just have to do it by hand as MCreator doesn't support that feature with it's procedures and all. That doesn't mean you can't and it's not allowed though.

You don't have the right to share modified minecraft's code. If he's asking to modify some code, I suppose he want to share his mod (with the modifications) which in this case, is not allowed. 

you dont have a right to share a modded minecraft version, but you do have a right to share something that mods an EXISTING version of minecraft

you can make and distribute something to mod minecraft's code, you just can't distribute a working version of the game; you can only distribute mods that modify an existing, legitimate copy of the game

so in short, you CAN make a mod that changes vanilla code without legal repercussions

Yes but how