They look very similar from a surface point of view so we have to dig a little deeper.
Using Richard Lawrence's Simple Key to British Rodent and Lagomorph Skulls we can understand the subtle differences we need to look for.
Best things to look for are teeth and the palate bone.
The end of palate bone is noticeably different. With the Bank Vole having a much deeper looking notch in the end. Whilst we dont have an example of the teeth the Bank Vole has two roots on the molars which fit into the jaw unlike Field Vole which has one unified base.
Additionally the incisors can be looked at as Field Voles tend to be broader.
Never used these guys for research because they’re dominantly your average coyote. Only three are the AZ subspecies, but they will be added to my information now.
I don’t have a big enough set up to take a good picture of them with proper lighting but eh, oh well