In case you didn't know, this argument has been easily refuted.
It argues that more animals die in growing plants than the animals that are killed for meat. This is partly true: many animals living in the ag fields are killed when the plants are harvested. However, we must feed the animals we eat, and, in fact, feed them 10 times as much plant matter as we would eat ourselves if we just ate the plants. (10-times is an average; some animals consume less, some consume more.) So we have to grow 10 times as much plant matter AND THEREFORE KILL 10 TIMES AS MANY ANIMALS in order to eat animals over plants. And this is without taking into account the number of animals being fed these plants...
So, while eating solely plants does often result in the death of nonhumanimals, eating animals will just as often require a far larger number of nonhumanimals killed in the process.
So, if we are trying to minize our harm, eating plants wins out. Unless, of course, you are eating wild game or 100% grass-fed animals, or other 'special' conditions are present.
If stuck with land such as above, it would only be sensible to raise crops suited for livestock, as well as the livestock to feed it to if space allows. If there's rights for the field mice, then does that mean the farmer who owns the land should let his family go broke and starve? At least the people who don't follow a strict vegan diet make use of the animals they kill (cows, sheep, chickens, etc.) by eating them. Strict vegetarians don't make any use of the animals killed while harvesting their food.
Also, if there's no moral "cost" to going vegan, then what about the controlled hunts to eliminate pests like deer from the fields? How about the disruption of that field's ecosystem by farmers introducing predators into the area who kill off smaller pests such as rodents and insects?
To say there's any sort of superiority morally to going vegan is pure hypocrisy.