Daisy has this vest in pink (see Avatar). It is very secure because it has velcro, plus a clasp, and also 2 rings to attach the leash. But I do think it would be a little uncomfortable in the hot summer.
For summertime, I have her wear the regular Puppia mesh harness. I just noticed they have a cotton one that would probably be great for hot days!
TheNewYorkDogShop.com