vishawaroopa of purush shukta is not dedicated to Vishnu or any other god. The description of Supreme creator ( Adi Purush) was used by vaishnavites later in Yajurveda and subsequently in other Vaishnav texts.
read urself !
Om sahasra shirsha purushaha
sahasrakshas sahasrapat
sa bhumim vishvato vritva
atyatishthad dhashangulam
(The Purusha (the Supreme Being) has a thousand heads, a thousand eyes and a thousand feet. He has enveloped this world from all sides and has (even) transcended it by ten angulas or inches)
purusha evedagam sarvam
yadbhutam yaccha bhavyam
utamritatva syeshanaha
yadanne natirohati
All this is verily the Purusha. All that which existed in the past or will come into being in the future (is also the Purusha). Also, he is the Lord of immortality. That which grows profusely by food (is also the Purusha).
does is ring some bells !
These verses do not stand in Isolation, they are similar to Mandala 7 which is the Vishnu Sukta which gives the same description to Vishnu. That is how we know Purusha Sukta talks about Vishnu.
Paro matraya tanva vrdhana na te mahitvam anv asnuvanti,
Ubhe te vidma rajasa pathivya visno deva tvam paramasya vitse,
Na te Visno jayamano na jato deva mahimnah param antam apa,
ud astabhna nakam rsvam brhantam dadhartha pracim kakubham prthivyah,
Irvati dhenumati hi bhutam suryavasini manuse dasasya,
vy astabhna rodasi visnav ete dadhartha prthivim abhito mayukhaih,
urum yajnaya cakrathur ulokam janayanta suryam usasam agnim,
dasasya cid vrsasiprasya maya jaghnathur nara prtanajyesu,
MEN cannot compete to your magnificence, who grows beyond all bound and measure with thy body.
Both thy two regions of the earth, O Visnu, we know: thou God, we know is the also the highest,
None who is born or being born, God Visnu, has reached the utmost limit of thy grandeur.
The vast hight of heaven has thou supported, and mapped and held earth's pinnacle securely.
Rich in sweet food we offer you, and rich in milk, with fertile pastures, you offer to do men service.
Both these worlds, Visnu, has thou mapped with your feet, and firmly fixed the earth with pegs around it.
You have made spacious room for sacrificing by generating Surya, Dawn, and Agni.
O Heroes, ye have conquered in your battles even the bull-jawed Dasa's wiles and magic.
Now go to the next chanda that talks bout Vishnu covering the three worlds in his strides.