About 200 dog lovers and about 50 dogs gathered in Vancouver, Washington, yesterday to protest a proposed ban on pit bulls, which is one of a number of restrictions on the breed being considered by the Vancouver City Council.   The protesters lined up along Mill Plain Boulevard, many of them carrying signs to protest the possibility of a ban on pit bulls. The issue has been hotly debated since a nine-year-old local boy was sent to a hospital after being attacked by three pit bulls outside a home on Evergreen Boulevard. The Council plans to discuss proposed restrictions at a work session sometime in August.