I read in Google groups that it doesn't. Can anyone with access to Vishnu Purana verify this?
Narasimha Avatara is mentioned in Vishnu Purana but it is not described in detail. You can see index of Vishnu Purana here.
Sage Parashara said Lord Vishnu would appear as Narasimha and would kill Prahlada's father. Lord Vishnu manifested before Prahlada and granted him that he would get Moksha. Thereafter Parashara said Lord Vishnu killed Hiranyakashipu.
Chapter 20, book 1 of Vishnu Purana says (translated by Horace Hayman Wilson)
इत्युक्त्वान्तर्दधे विष्णुस्तस्य मैत्रेय! पश्यतः।
स चापि पुनरागम्य ववन्दे चरणौ पितु:।।२९।।
तं पिता मूर्द्धन्युपाघ्राय परिष्वज्य च पीडितम्!
जीवसीत्याह वत्सेति बाष्पार्द्रनयनो द्विज। ३० ।
प्रीतिमांश्चाभवत् तस्मिन्ननुतापी महासुरः।
गुरुपित्रोश्चकारैवं शुश्रूषां सोऽपि धर्म्मवित्॥ ३१॥
पितय्र्युपरतिं नीते नरसिंहस्वरूपिणा।
विष्णुना सोऽपि दैत्यानां मैत्रेयाभूत पतिस्ततः॥ ३२॥
ततो राज्यद्युर्ति प्राप्य कर्मशुद्धिकरीं द्विज।
पुत्रपौत्रांश्च सुबहूनवाप्यैश्वर्य्यमेव च॥ ३३॥
क्षीणाधिकारः स यदा पुण्यपापविवर्जितः।
तदासौ भगदद्यानात् परे निर्वाणमाप्तवान्॥ ३४॥
Thus saying, Visnu vanished from his sight, and Prahlada repaired to his father, and bowed down before him. His father kissed him on the forehead', and embraced him, and shed tears, and said, "Dost you live. my son?" And the great Asura repented of his former cruelty, and treated him with kindness; and Prahlada, fulfilling his duties like any other youth, continued diligent in the service of his preceptor and his father. After his father had been put to death by Visnu in the form of the man-lion. Prahlada became the sovereign of the Daityas; and possessing the splendours of royalty consequent upon his piety, exercised extensive sway, and was blessed with a numerous progeny. At the expiration of an authority which was the reward of his meritorious acts, he was freed-from the consequences of moral merit or demerit, and obtained, through meditation on the deity, final exemption from existence.