I originally used the tunnel set-up with back-to-back mouse traps on the above ground trails. As in all vole trapping, drill a hole in the trap and stake with a nail. No bait needed. However, I ran into some of the areas like you mentioned with just a series of holes, and no trails or runways above ground. I have been having fantastic results using this set up. Using my mole knife (Fiskar's garden knife), I cut a wedge at the hole, creating a 45-degree trap bed. Cut the width of the mouse trap, making sure to have clearance for the striker bar to clear the hole. Stake down with a nail. Set trap, pick up dead vole next trip.
I added a step to this system to cut down on the number of traps. Some areas will have 75 to 100 holes or more, and there's no way to set them all. However, I set about 1 out of 5, and use the landscape sealant (black spray foam w/applicator gun) to squirt a blob into all of the holes I didn't set. Yes, voles could chew through it... but they don't. They follow the air flow or daylight, and go right to the openings with the traps.
This one shows the wedge I cut out (wedge at left). Trap pinned to *ramp* with 16p nail. Note squared-off cut-out at top of hole so bar clears.
Close-up of a vole in trap. Usually a perfect neck catch as they come out of the hole.
Section of ground showing multiple holes, some with traps, others sealed with foam.