There are several Kavachas associated with Mahāviṣṇu that are seen in the various purāṇas. Of these, Nārāyaṇa Kavacha which occurs in the sixth skandha of Viṣṇu Bhāgavata Upapurāṇa is the most popular. This is especially true with those that scoff at rituals proclaiming their loyalty to either Jada Vedanta or to some kind of medieval Bhakti cult (to put it less correct politically, the Karma Bhrashtas).
However, the knowers of Mantra śāstra have traditionally recited the Viṣṇu Kavacha from Viṣṇudharmottara Purāṇa. This purāṇa deals extensively with solutions to afflictions by various kinds of Grahas and the Kavacha also occurs in this context. This Kavacha, much older than the Purāṇa, seems to have been inserted later into the Viṣṇudharmottara, on account of its massive popularity with both Smārta and Pāñcarātrika practitioners of Mantra śāstra. Again, many interesting details can be discussed regarding its origin, but we shall save that for a separate topic.
A relatively śuddha pāṭha of this Kavacha has been uploaded to our site. Access the file here.