That depends on how deep your template interferes with the html of spark.
Vue is refrenced in the html in many many places (variables, directives, inline templates etc). So as long as you mainly change around css definitions you should not have many issues (unless you have an overlap in class names between your template and the ones used by vue for funttionality).
I decide to dive head first and just try it out and well it wasn't to bad..
the main issue I have is the template I chose has bootstrap/jquery in it as well, and there is conflicts between the app.js and my templates...
but other then this, seems not to bad..