I think to him, the terrace wedding has sanctity. And thats why he says he is married. But he also knows legally their marriage didnt complete. Thats why he is not contesting Mannat's marriage and going ahead with his engagement. Under emotions, he says he is married because thats how he feels!!
I could be totally wrong and this could be the confused writers, who are still figuring out what way they should go.
bold... this makes sense though...
both terrace and social wedding were incomplete.... the wedding was complete in installements though ... 7 phere and varmala 
the other angle is Vik is operating from a grudge... he feels she abandoned him... which is not the case but he doesnt know that. And there is a great secrecy over what happened to those papers.
Since looking at how angry Vik is... usne har baar Mannat ko sunaya hota ki usne papers sign kiye hai... which he is not doing 
Vo GD itni baar bola hai ki vo divorce ke liye na bole aisa ho nahi skata 

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