In a large saucepan, bring 3 cups raspberries, 1 cup sugar and 1 cup water to a boil over medium-high heat, stirring until the sugar dissolves. Reduce the heat to low and simmer gently, stirring occasionally, until the mixture thickens to a syrup, about 20 minutes. Strain through a finemesh sieve set over a medium bowl, pressing on the raspberries; discard the solids. Stir the lemon juice and vanilla into the raspberry syrup.
Place the remaining 1/2 cup raspberries in a medium bowl. In a small saucepan, bring the remaining 1/4 cup sugar and 1/4 cup water to a simmer over medium-high, stirring until the sugar dissolves (do not boil). Pour the mixture over the raspberries and let stand until they release their juices, about 30 minutes.
Divide the raspberry syrup and seltzer among 8 tall glasses; stir gently to combine. Add 2 scoops of ice cream to each glass; garnish with whipped cream and the raspberry topping.