Buddhist higher ordination is not a vow in the western sense. The candidate is admitted to a training rule and undertakes to keep it: 227 clauses for a Theravada monk, 311 for a nun, more in some other recensions. The rule is presented in the Vinaya as case law, a sequence of incidents followed by the ruling each provoked, rather than as legislation handed down.
Because it is an undertaking rather than a lifelong vow, it can be relinquished. A monk may formally disrobe and return to lay life without disgrace, and in Thailand, Burma, Laos and Cambodia ordaining for a rains retreat or for a few weeks is ordinary. Many men do it once in their lives, some as boys, and the merit is understood to accrue to their mothers. This is very hard to map onto a Christian frame in which leaving is a failure requiring dispensation.
The unresolved question concerns women. Full ordination for nuns lapsed in the Theravada countries centuries ago and the Vinaya requires existing nuns to be present at the ordination of new ones, which appeared to make revival impossible from within. The lineage survived in East Asia. In December 1996 ten Sri Lankan women were fully ordained at Sarnath by Sri Lankan monks assisted by Korean monks and nuns, and a much larger international ordination followed at Bodh Gaya in February 1998 under Taiwanese auspices, with a quorum of nuns from Taiwan.
Sri Lanka now has a substantial community of fully ordained nuns. The Thai and Burmese monastic hierarchies have not recognised these ordinations, and the argument turns on whether a lineage of a different recension can supply what the Theravada one lost, and on whether the requirement can be dispensed with at all. Scholars within the tradition have argued in print that the objections do not hold on Vinaya grounds. Women who ordained in Thailand have faced formal sanction. The dispute is live and it is conducted in the technical language of monastic law rather than in the language of rights, which is worth knowing before reading either side.