First of all Vin has a history of playing lead. Showing him as Pragya's fake lover gives him more footage rather than being a help in the background.
If Nikhil or some other guy was plotted, Abhi may not ship Pragya with that guy as strongly as he may ship Pragya with Purab. Abhi believed once upon a time that Purab is the best guy around and wanted him to marry Aaliyah and now he keeps Nikita in his good books and may be shown to help Pragya-Purab marry and that leading up to lots of mess.
Aaliyah has a master stroke here too with 'not-morphed' pics and to make Nikita in a status of Bhabhi for Abhi. Even if Abhi feels for Nikita romantically, he will step back given Purab-Nikita love.
And CVs get a hold to create drama in wedding related events for Purab-Pragya too and to throw AbhiGya dream and accidental lollipops and may be some angst too!!
No I am not defending this track and would love for it to go some other way and not mess with a bro-sis bond..but just saying!