I wanted to create a custom loot table but first i wanted to run a test to see if it works. So for now i gave the vex the loot table of the slime. I placed the new vex.json w/o modifying any part of the code in the folder described in the current 1.13 data pack wiki. went in game killed a few vex and nothing happens.
..The data pack is working because all the custom crafting that i have added works fine.
Solved! I found by watching a Xumavoid video that to overwrite pre-existing loot tables your loot table must be in the following folder. With the namespace required to be "minecraft" Not as seen in the information provided by the wiki. That kinda of information needs to be added to the wiki.