While I confess I am not totally shocked by the responses you have gotten from the police, I believe the could and should do something. My recommendation would be to talk to an attorney who has experience in handling police misconduct cases. (civil rights violations for example). You need help in getting local law enforcement's attention. With respect to the lease, the stalking situation is basically irrelevant since the landlord has nothing to do with it nor any obligation under the lease to prevent it. Not sure that moving is even an answer to the problem, but you need to examine the lease and consider discussing that as well with an attorney. The lease question is pretty basic stuff that the attorney you secure to deal with the real problem can probably help you out. Good luck.
Get a temporary restraining order. Make sure it is served properly. Then if she returns, call the police.
Your landlord is not responsible for that, and it doesn't have anything to do with your lease.
Clark County, NV practitioner.
I changed your practice area to landlord-tenant because this is not really a criminal defense question (you aren't being charged with a crime).
A stalking order requires at least two unwanted contacts and you can seek one yourself through the courts, rather than the police. If you can show proof of the threatening messages and vandalism, that will help your case greatly. But remember, it has to have happened at least twice and it has to be unwanted. Has your boyfriend ever asked her not to contact you anymore? Have you made police reports? Have you considered installing cameras like the other poster suggested?
I agree that you probably cannot get a restraining order because of the lack of a familial relationship. There might be an option here if she and your boyfriend have a child together, "cohabitated," or were in a sexually-intimate relationship within the past 2 years. If you want to read a bit more about whether he (it wouldn't be you) qualifies for a restraining order, here's a link to a brief guide I wrote on the topic: http://www.avvo.com/legal-guides/ugc/restraining-order-101-family-abuse-prevention-act
I also like the other posters idea about asking the landlord to install security cameras for you and having an attorney review the lease to see if any of the provisions help you. You could probably get an attorney to talk to you about all three options. Many attorneys will do a consultation free of charge if you ask them to. I for one would be happy to discuss this case with you because you raise some interesting issues.
My responses to posts on AVVO are not legal advice, nor do they create an attorney-client relationship. In order to provide true (and reliable) legal advice, an attorney must be able to ask questions of the person seeking legal advice and to thus gather the appropriate information. In order for an attorney-client relationship to exist, you and I both have to agree the the terms of such an agreement.
The landlord tenant portion of the answer is found in ORS 90.453 "Termination by tenant who is victim of domestic violence, sexual assault or stalking"
This section of statute explains exactly what has to be given to the landlord in the form of a verification statement proving the stalking. It is rather specific in its requirements and anyone seeking to help you get a restraining order should also review this statute up front to make sure they lay the groundwork to satisfy the requirements of this section.