I definitely do not think the millennial vote is up for grabs. Being a millennial myself I would say the vast majority of my cohorts (ok maybe 60%) do not care at all about politics and when they do vote they do so because it's "cool" or "hip" as the media makes it out to be and they usually go for the candidate the media frames the best (like Obama in 2008/2012). As of now I think a good 60% of us need to read up more on politics and less on Hollywood.
The other 30-40% I'd say is split between 80-90% being left-leaning and 10-20% being right-leaning (and I mean really moderate. I'd confidently say homophobia and opposition to same-sex marriage/racism will probably be dead in 10-20 years when my generation takes over).
There's more right leaning millennials than you think (myself included)