Date her before she finds somebody else?

I'm sorry to say this because you seem to really like her but I don't think she is interested. If you two have been so close going to movies, vacations, etc.. then she has had plenty of time to get to know you. If she has decided on a dating site she is ready to start dating again then I would assume she would approach you first. I don't know she sounds like a good friend (as long as all the outings are split) but it doesn't sound like she is interested. She knows you tried to ask her out before so...My advice you have two options. Let it go and find someone else or go for it and ask her out one more time. If you ask her out again just be prepared the worst and don't let it get to you too much. I wish you goodluck however it may work out.