If and elf and a vampire had a baby, would it mostly be elf or vampire or a mixed of both?

Probably more vampire but it would have the elf ears

It would be a vampelf. Ugly little critters, not intimidating in the slightest. They usually spend all their time skipping around the forest and making bat-shaped cookies in a treehouse.

Seriously, even those Twilight vampires laugh at vampelves, and Twilight vampires are total pantywaists.

An elfin vampire: the Leanan Sidhe.

First things first - neither creature has actually existed.

Second, if you want to go by actual legends, the fae would never associate with such a thing, and the mythic vampires were the risen corpse of a human that caused the deaths of its family and friends before returning to the grave, usually for good. Dead bodies don't procreate. A male vampire has no viable sperm and a female vampire cannot become pregnant - they are dead.

If you want to look at modern fantasy where everyone writes vampires as something far removed from their mythical antecedents, then by all means, follow Meyer's ridiculous path and imagine whatever you like.

it would only be a - figmentation of one imagination

might be a mutant monster with no teeth and one eye

Worry about it when it happens. Which it never will.