I actually think you are spot on! For this usage of the genitive, there is little need to match the genders of the nouns. Both Aurora and votum are their own, separate entities, so they do not need to match. Votum Aurorae is therefore a good translation.
Now, I can't really make too much of a comment on how poetic it is. I, personally, think it is fine. But to change it up, you could go a couple of directions. Aurora was the goddess of the dawn, so one could try to find other options that mean "dawn," "sunrise," etc. You could also try to find adjectives that mean "of the dawn," in which case you would need to match the gender to votum. One could also find other substitutes for votum. Below are some options I found. It is really up to you to determine what is most poetic.
Nouns = "Dawn," "Sunrise"
- ortus, ortus, m.
- Eosos, Eosi, f.
- oriens, orientis, m.
Adjectives = "of the dawn," "of the sunrise,"
- antelucanus, antelucana, antelucanum
Nouns = "vow," "promise"
- debitum, debiti, n.
- convotus, convoti, m.
- promissio, promissionis, f.
- promissum, promissi, n.
- pollicitatio, pollicitationis, f.
- pollicitum, polliciti, n.
- stipulatio, stipulationis, f.
- credentia, credentiae, f.