I bought mine preowned at www.encorebridal.com I could not recommend them enough they have been a life saver my gown is beautiful and I even bought my veil through them. I love them!
I didn't buy my dress online, but I did ask the same question about online reliability. amcs76 made a good suggestion if you're looking for a specific designer dress - to shop online at a store that is connected with an actual brick and mortar shop. That way you have a better chance of getting an actual designer's dress rather than a knock-off. Check my blog for the post and see her comments on it.
Incidentally - the dress you have pictured was the exact dress I was looking for too! I ended up getting Maggie Sottero's Samara instead of the Vincenza, but it was close. That is a fabulous dress!
I bought my dress online at www.bridalonlinestore.com and saved a bundle. Paid $250 for a brand-new gown. Beautiful dresses, i thought, for brides, bridesmaids and flowergirls. Vast selection in some hard to find sizes (0 to 30 I think) and ship quick!! Received mine in 10 days (shipped to Canada)
I just purchased mine online at bestbridalprices.com. Not only is it free shipping and no tax, you also get a free veil and shoes!!! And mine is a Maggie Sottero!
Incidentally - the dress you have pictured was the exact dress I was looking for too! I ended up getting Maggie Sottero's Samara instead of the Vincenza, but it was close. That is a fabulous dress!