Welcome Dost...
What I would suggest to you is that the VO is looking for things that show your relationship is GENUINE and CONTINUING. Things that are common in any normal love relationship:
Communication: how often do you communicate and what do you talk about?
This can be more difficult than you think. My husband and I did most of our talking on webcam and didn't have a lot of deep conversations by written text. You want to provide printouts or screenshots that show all your communication on Skype, text, email etc. But just to print out 100s of pages is not enough. They are looking for content. Especially if there is an age, religious or cultural differences, you want to show that you've talked about it and the conclusions you come to as a couple. If one spouse does not want to or is unable to have kids, it would be helpful to show your conversations around this also. Written communications that show how you're relationship developed and how you fell in love are important. And any love letters or cards should be photocopied.
Visits: has the sponsor visited the applicant?
Copies of airplane tickets, hotel receipts, car rentals etc to show that being together in person is important to you.
Wedding ceremony: Very important to have lots of photos of your marriage showing you followed the traditional norm for your wedding ceremony for the culture and that family and friends were present in support of your marriage.
Financial interdependence: Are you sharing finances? Copies of money transfers, joint bank accounts or credit cards, if one of you has put the other on your work benefits or life insurances help show you are working together financially and trust one another. Also any gifts sent if you still have the shipping label.
Photos: meaningful pictures showing your travels together, honeymoon, pictures with friends and family are very important to show that your relationship is public knowledge and that your friends and family support you.
This is about all I can think of at the moment....hope it helps!