I hope that Nessie would be able to have kids. I don't know though. You can calculate the species of the kid in a few ways.
If you consider Jake as a half wolf/half human and Nessie as half vampire/half human, then the kid would be 1/4 wolf, 1/2 human, and 1/4 vampire.
If you consider Jake as a full werewolf and Nessie as half vampire/half human, then the kid would be 1/2 werewolf, 1/4 human, and 1/2 vampire.
I don't know how the child would behave though. Would he/she turn into a wolf or drink blood or something?
I think that your opinion on whether Nessie can have kids depends on whether you think that when Nessie is grown, she'll "turn" into more of a vampire when she grows up.
I guess my answer to your question is: I HAVE NO ANSWER!