sorry 2door, i forgot to answer your question!
you have to delete the voles. piping your engine exhaust to the hole is simply the most effective way, but you have to find their holes, which could take a while. to protect your junipers buy a plastic tree collar and wrap it a million times around the trunk of your juniper if that is the part attacked. if it is the branches, spray castor oil on them, and place traps all over the place, baited with peanut butter. either that or get the ultimate vole killer: kitty! or your 12 gauge and get your sodas and your armchair to hang out nearby. you'll blow away their miniature minds for sure!
sorry! i don't like voles.