Hi,
ok,
First thing, I updated Vuforia package on the Ecosystem to use the latest PlayMaker Utils, so I strongly recommend you update your package.
second, with that package, it works fine with Vuforia 7.1.35 on Unity 2018.2f5
I have had many issues with the 2018.1 release and I would suggest you move to .2.
can you confirm that on a fresh project in 2018.2 it works? if that's the case, it's a case of cleaning up your project and install the packages fresh. Let me know if that still doesn't work after that.
Bye,
Jean