Are "precepts" as they are often known rules, vows or something else?
Looks like it depends on who's doing the translation and also which tradition is transmitting as some are using these terms interchangeably and have their own interpretation on what constitute vows, precepts, rules...
This is why lineage for precepts is essential. You cannot just pick up the Vinaya or Bodhisattva precepts and practice from there.
I am not sure if that's entirely true. The Mahayana Brahma Net Sutra does allow one to undertake the Bodhisattva Vows with certain stipulations under certain conditions...
http://www.ymba.org/bns/bnsframe.htm23. On Teaching the Dharma Grudgingly
After my passing, if a disciple should, with a wholesome mind, wish to receive the Bodhisattva precepts, he may make a vow to do so before the images of Buddhas and Bodhisattvas and practice repentance before these images for seven days.
If he then experiences a vision, he has received the precepts.
If he does not, he should continue doing so for fourteen days, twenty-one days, or even a whole year, seeking to witness an auspicious sign.
After witnessing such a sign, he could, in front of images of Buddhas and Bodhisattvas, formally receive the precepts.
If he has not witnessed such a sign, although he may have accepted the precepts before the Buddha images, he has not actually received the precepts.
However, the witnessing of auspicious signs is not necessary if the disciple receives the precepts directly from a Dharma Master who has himself received the precepts. Why is this so? It is because this is a case of transmission from Master to Master and therefore all that is required is a mind of utter sincerity and respect on the part of the disciple.
If, within a radius of some three hundred fifty miles, a disciple cannot find a Master capable of conferring the Bodhisattva Precepts, he may seek to receive them in front of Buddha or Bodhisattva images. However, he must witness an auspicious sign.
I was attending a 4 day teaching recently on Atisa's Bodhipathapradipa and the presiding Rinpoche mentioned that only in acute and exceptional circumstances (like in times of war, famine, sickness, disasters, upheavals, totally unable to find a proper Guru/Preceptor) that one can undertake the Bodhisattva Vows by making offerings to the Triple Gem and oneself motivated by Bodhicitta and a sincere aspiring mind. One need not wait for any auspicious signs as one's pure motivation suffices unlike the stipulation of witnessing an auspicious sign in the Brahma Net Sutra. He repeated that this method is only for the exceptional cases.