Complex sentence: The main donor to the VPC is the Joyce Foundation, who additionally supports other pro-gun control organizations, including the Mayors Fund to Advance New York City and the Illinois Council Against Handgun Violence.
Split into simpler sentences: The main donor to the VPC is the Joyce Foundation. This foundation is also a supporter of other pro-gun control organizations like the Mayors Fund to Advance New York City and the Illinois Council Against Handgun Violence.