What would you expect/want if this was the other way around (you giving her a gift she has no use for)?
Given your 1.5 years together, you should be able to safely let her know that the vest is not an item that you have use for, and why. If that sends her into a fiery tailspin, you have bigger problems. If you are reasonable folks, she'll get over it and there will be no lasting issues. And hey, she'll have a more accurate understanding of you as a runner, and maybe even get you something useful down the road. See if she can return it or if you can sell it online. Time heals all wounds. Best of luck to you.
Alternatively, I second the suggestion that you take up distance running or use it for hikes. Brownie points for showing her photographic proof of you using it - better yet, post on social media, caption=rave about her thoughtfulness.