The supreme abode of Vishnu is non-dualistic in nature as mentioned in Vishnu Purana. In my reading, I found that Vaikuntha word is absent in Vishnu Purana. And while reading it I came across this verse.
यत्तदव्यक्तमजरमचिन्त्यमजमव्ययम् । अनिर्देश्यमरूपं च पाणिपादाद्यसंयुतम् ॥६६॥
विभुः सर्वगतं नित्यं भूतयोनिरकारणम् । व्याप्यव्याप्तं यतः सर्वं यद्वै पश्यन्ति सूरयः ॥६७॥
तद्ब्रह्मतत्परं धाम तद्धयेयं मोक्षकाङ्क्षभिः। श्रुतिवाक्योदितं सूक्ष्मं तद्विष्णोः परमं पदम् ॥६८॥
That which is
imperceptible, undecaying, inconceivable, unborn, inexhaustible,
indescribable; which has neither form, nor hands, nor feet; which is
almighty, omnipresent, eternal; the cause of all things, and without
cause; permeating all, itself unpenetrated, and from which all things
proceed; that is the object which the wise behold, that is Brahman,
that is the supreme state, that is the subject of contemplation to
those who desire liberation, that is the thing spoken of by the Vedas,
the infinitely subtle, supreme abode of Vishnu (तद्विष्णो: परमं पदं)-- Vishnu Purana - 6.5.66-68
This verse is quite explicit and it leaves no doubt that formless Brahman is the supreme abode or state of Vishnu.
The above translation is based on Sanskrit-Hindi Vishnu Purana
Source: English translation.