What happens if you are a half werewolf-half vampire?

Well,if you mean someone born from a wolf and a vampire parent, that probably cant happen unless maybe if the vamp is a hybrid like Renesmee ( Stephanie Myers wont officially reveal if she can have babies.) We do know that female vamps cant have babies, and it is implied that female wolves cant either, since Leah stopped having her period after she transitioned. Therefore, there can be no crossbreeding.
If you are talking about one species being turned by another, I have a pretty long answer. In the Twilight universe we really cant answer this because we dont know much about their version of werewolves, since it is revealed in Breaking Dawn that Jacob and his friends are not werewolves, they are shapeshifters. This means that they are born this way, and cant turn anyone else into werewolves, so they cant create vampire,werewolve hybrids. It the Twilight universe vampire venom is poisonous to the shapeshifters, so they would most likely die instead of becoming a vampire, so the vampires cant create hybrids using them either.

In one of my Twilight fan fictions, two of the characters were half-vampire/half-werewolf. My guess is that a half-vamp/half-werewolf would be able to shapeshift into a wolf, but his/her diet would (preferably) consist of blood rather than actual food, even though he/she would be able to eat human food if they really wanted to. I also think that they would have the same abilities as any normal vampire, such as running at a super human speed. Their body temperatures would probably be in between freezing cold and very warm, so most likely they'd have an average human body temperature. Their skin would also probably be in between tan and pale, if not one of either extremes. Hope this helps answer your question!
: )

Tat would be difficult. I guess you'd be a hybrid; see tyler lockewood.

ha?that's odd.

ha?that's odd.

THE are both vampire and werewolf and have to live with it whether they like it or not

Well then the baby would be both half vampire and half werewolf and it would be considered a Hybrid. Like Tyler Lockwood from the Vampire Diaries or The guy from teh movie the Underworld:)

I would try to find a cure cause i really dont like being a werewolf but a vampire that would be great!

With Stephenie you never know. If Jacob and nessie were to have kids they would not be wolf/vampire. Like Billy it would skip a generation and it would be different. The Volutri will not allow it.

well if nessie can't get pregnant then one way a vamp/wolf crossbreed could happen is for a male vampire and one of the human women from the tribe to get together the baby would have vampire blood and werewolf blood in the books that said the boys started turning into wolfs because the vampires was in town so the vamp blood in the baby would couls the wolf blood to react... it's hard to tell if the baby would be more powerful than the vampires and wolfs being a cross of the two or if the two bloods would just kill the baby "vampire venom kills wolfs"

I would say it is possible but the only 2 that can do it is if Bella's daughter and Jacob have a kid because vampires cant have kids but Bella's daughter is half human half vampire so she has human traits too so she should be able to have a chilled and seance Bella's daughter can walk in the day and eat human food and blood it is an unknown mixture but most likely it would be a vampire able to drink blood and a shape-shifter/werewolf able to eat human food he or she might have 2 ability's a normal vampire ability like read mines or a shielder like Bella etc. and the ability to turn to wolf form and the chilled will be a vampire all her live like Bella's daughter and when she stops ageing at maturity which is 17 and then she/he will get her/his ability to turn into a wolf and seance he/she is human, vampire, and werewolf/shape-shifter she/he will most likely age normal but may be 2 years faster than a normal person. like every year she/he ages 2 times a year instead of once a year he/she in human form could have brown eyes and black hair seance Jacob and Bella's daughter has brown eyes and black hair and in wolf form he/she will have ether gold, red, or black eyes depends on what he/she eats or drinks and with brown fur and a body temperature of a normal human and able to go in the hottest temperatures and the coldest temperatures and stronger and faster than a vampire and a werewolf/shape-shifter and the hybrid 5 scenes will be better than a vampire and a werewolf/shape-shifter and maybe have a 6th scene and the 6th scene would be smartness