A fire in my neighbors house damaged the vinyl siding on one side of my home. Who is responcable to replace my siding, and how much siding .

Since the fire was at your neighbor’s home, the insurance company for your neighbor should pay for the damages to the siding.  It is possible that  your insurance company will pay for the damages and then subrogate against the other company.  However, if you decide to go that way, your deductible will apply until your company can get reimbursed. 

The amount of siding to be replaced is dependent upon whether the siding can be matched.  If it cannot be matched, then the insurance company will have to replace at least the entire side, and possibly the entire house. 

You should contact your neighbor’s insurance company and discuss the entire issue.  They should be willing to work quickly with you.  If not, talk to your insurance agent and see if you can file a claim with your insurance company.  However, the least expensive way for you is to work with the other insurance company.