This is the only method that has worked here after investing lots of $$ on traps, bait, etc.
STEP 1 - You need to determine which entrance/exit holes are active. To do this sprinkle some poultry grit. fine gravel over holes.
Vole Hole
STEP 2 - Check holes in a day or 2 to see if the gravel has shifted. If yes, it's active.
Active vole hole (Poultry Grit shifted.)
STEP 3 - Set a snap trap and cover with pot anchored with bamboo stake so you don't injure roaming animals.
You should be able to fairly quickly check pots by looking down through hold in pot.
The snap trap shown in photo kills instantly so there's no suffering.