The .c2d file is the projet file itself, the one you open in Carbide Create to edit your design and toolpaths.
For adding tools to the CC library, here’s a handy tutorial from @WillAdams:
Is your Vbit significantly different in diameter than the one you selected in the existing tool library ?
Now, if this used to work and you just recently started to have this problem, it may be something mechanical that went a little loose. Do you have a belt-driven Z axis or Z-plus ? When the machine is turned on and you try to wiggle the router up and down manually, do you feel any play ?