What will happen if a vampire is having sex with another vampire ?

If they are good at it, they will drive each other batty.

If you imagine mythical creatures, you can imagine them doing anything you want to imagine.

They will make a baby vampire.

According to Stephenie Meyer, author of the Twilight saga series, they just have sex.

They would closely observe the no-biting rule, the no-afternoon-quickies rule, and the no-garlic-breath rule.  They'd probably be limited to the cowgirl, the T-square and the dirty Santa, because missionary is for Christians and doggy style is for werewolves.

And if they don't use proper protection, one of them might become a mummy.

If you mean the little bats, the result is often a baby bat.

If you mean the 'human' kind of vampire, they're purely fictional so what will happen is entirely up to the writer of that fiction.

they make baby vampires

Vampires, do not exist, but vampire bats do however exist. If vampire bats have sex with each other, they then produce offspring, and like every other mammal, they nurse their young with milk. The baby bats drink milk, until they become old enough to live off blood just like their parents. I'm sure it would work the same way with "real" vampires if they existed.

If neither uses birth control, then in nine months there could be a baby vampire.

Vampires are imaginary so, no problem.