I have an idea!

How about setting traps baited with chillies, if the voles have a taste for them.  If the voles can scramble up the smooth sided pots, they might just climb up two pots unless you put, say, an upturned large saucer with a lip inbetween the two pots to make a barrier.
Apologies for the drawing,

Tony Hart's got nothing on you Jen

I had to figure out how to draw with my finger on my tablet, then get it onto a picture to enable to post it .  We got there in the end.  Finger  painting next