Any of those manufactures is pretty reputable. I would have no issues buying from any of them. However, I will make comments on the specific models you listed.
MSI HAWK: Best cooling and potentially better overclocks, cost less than the Vapor-x
Sapphire VaporX: These models are ALWAYS overpriced from Sapphire. Still Sapphire is a good brand.
XFX: doesn't have an aftermarket cooler for the 5770, they use the 2 reference coolers and that is it. However, they offer the best warranty.
Any 5770 will allow a moderate overclock. However, if you are looking at the max OC you can get, I'd swing toward the MSI HAWK.
http://www.tweaktown.com/reviews/3124/msi_radeon_hd_5770_hawk_1gb_video_card/index.html