I think it would be ideal if we all had committed vegan partners, committed both to us and to being vegan. Considering the relatively small population of existing vegans it makes sense that we'll have to (get to!) "convert" some folks as we move forward. If your friend is otherwise wonderful plus is willing to become a vegan for you and for however long your relationship lasts, that sounds like a close runner-up! Better for her to be vegan for you and for however long then to never be vegan at all. It is also quite possibly better for you and her to enjoy a partnership if her not yet being vegan is the only major concern that you have - rather than waiting for an even more ideal person coming along.
That said, I of course also fully understand you wanting her to choose to be vegan of her own accord / for the animals, etc. Have you shared with her your reasons for being vegan and how important it is to you, not just for yourself but for her? Perhaps more importantly, have you listened to her reasons for not yet choosing to be vegan herself? While I appreciate your pragmatism and maturity in considering what may occur after your relationship possibly (probably considering present trends) dissolves in the future, it is also somewhat concerning that this seems to be a notable concern if you are considering marriage... I also noticed that you write: "I don't want food to become a source of tension" - of course, being vegan isn't at all just about what we eat so I also wonder how open your friend is to having a vegan lifestyle beyond food. Those are my late night thoughts - hopefully you find them at least somewhat helpful.
Please let us know how things go for the tow of you!