The biggest Vocaloid ONLY event, called 'The VOC@LOID M@STER14' was held on November 14th in Tokyo, at an exhibition center called Sunshine City in the Ikebukuro district. Doujin events called 'ONLY' are focused on a single category of items for exhibition or sale. I went straight there!
The venue was crammed with around 500 booths vocaloid content developers. In addition, over a thousand people were waiting to enter in from early in the morning to buy what they really wanted. Sought-after items were sold out as quickly. A limited supply of 5000 catalogs priced at $10 each (combined with admission fee) were sold out in advance. The catalogs were in such high demand that were sold for as high as $150 on Yahoo auctions! Unfortunately, if you didn't purchase a catalog first, you had to wait until after people who had the catalog were allowed to enter.
Once the door opened, the venue became like a crowded train. Soon there were long lines in front of booths run by vocaloid music producers, which were marked by the letter 'P'. There were also booths run famous painters, as well as by companies like Yamaha that make vocaloid software and hardware. (Sales clerks were beautiful cosplayers!) Many vocaloid music groups introduced their original CDs, photo collections and artworks. These group members also appeared in cosplay.
Here is the crowd shot from the event that I managed to take. Have fun!