THis is one of those cases where we can't just put in real world mythology into the already well established TES mythology. Just because X is a certain way in our world, doesn't mean it has to behave the same way in TES. Clearly, influences exist. But don't get bent out of shape just because it isn't a perfect mirror of what you expect to see.

For me, it just doesn't seem right in the "time period" we see in The Elder Scrolls. Vampires that turn into bats belong in the 18th, 19th century in my mind... you know, this gothic feeling is missing in a medieval-style fantasy world like Tamriel. Or, I don't know, it might even work in some medieval-style games, but the visual style and also the atmosphere would have to be drastically different. Just... yeah, more gothic. Crooked buildings, a civilization suppressed by both the rulers and the cleric, strong superstition against magic... in such a world, I can see vampires that turn into bats, fly through open windows at night and feed on the poor maiden asleep in her nightgown.

This notion probably seems completely far-fetched and weird. I also have no idea what vampires in Skyrim *should* be like.

I'd rather them shimmer in sunlight and knock up teenage girl.

If you were going to have vampires transform into anything, I wouldn't pick a flying rat. Choose something that's more intimidating and powerful.

Bats are not scary, they are pretty cute. The only ones that svck blood are in South America, appropriately called vampire bats, and I don't think there's been a confirmed case of someone dying from blood loss (maybe from transmitted disease, but the bats usually feed off of cattle). The rest eat insects, fruit, and nectar.